How to align Radio button at the center of the screen

Try android:gravity="center" in the xml for each RadioButton.

I see you're wrap_content, so theoretically that shouldn't do anything, but it looks like the icons are evenly divided, maybe the RadioGroup divides the space evenly so using android:gravity will have an effect.

Worth a shot I guess.

EDIT

Try this:

  <RadioButton android:id="@+id/allcontacts" 
  android:layout_width="0dp"
  android:layout_height="wrap_content"
  android:layout_gravity="center"
  android:marginLeft="20dp" 
  android:gravity="center"
  android:layout_weight="1"/>

You will have to play with the android:marginLeft="20dp" to get it properly centered, but that'll let you do what you're looking to do. I copied your layout and tested this time, so I know it works! :)

I had similar issue before, I figure it out, so I try to explain it for you.

above is my app screenshot, as you can see, I also did the same, I had a menu align screen bottom, they're a RadioGroup and three RadionButton inside it, I want each menu icon align center for RadionButton, the shadow image(9-patch) shown together when Button check event trigger.
At first, I used android:button attribute refers to a drawable selector, the selector has two drawable state between check and uncheck, but I cannot align the menu icon center and make the shadow image fill up by RadionButton, they looks like it :

I tried included android:layout_gravity=center and android:gravity=center with the RadionButton, but it also didn't got effect. after that, my workmate told me the android:button attribute is refers to foreground rather than background, then I use android:background instead of android:button and it worked, below is my code :

<RadioGroup android:layout_width="match_parent" android:layout_height="60dp"
        android:orientation="horizontal" android:background="@drawable/menu_bg">

    <RadioButton android:layout_width="0dp" android:layout_height="match_parent"
                 android:layout_weight="1" android:button="@null" android:checked="true"
                 android:background="@drawable/menu_item_selector" />

    <RadioButton android:layout_width="0dp" android:layout_height="match_parent"
                 android:layout_weight="1" android:button="@null"
                 android:background="@drawable/menu_item_selector" />

    <RadioButton android:layout_width="0dp" android:layout_height="match_parent"
                 android:layout_weight="1" android:button="@null"
                 android:background="@drawable/menu_item_selector" />

</RadioGroup>

the menu_item_selector is important thing because it had gravity setting :

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/menu_item_layer_list" android:state_checked="true" />
    <item android:drawable="@drawable/menu_item_layer_list" android:state_pressed="true" />
    <item>
        <bitmap android:src="@drawable/menu_item_off" android:gravity="center" />
    </item>
</selector>

cause I had two images to draw when user turn Button state is checked, the shadow background image and icon active state image, so I used layer-list to implement it, the active icon also had gravity setting, below is menu_item_layer_list code :

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/menu_bg_pressed" />
    <item>
        <bitmap android:src="@drawable/menu_item_on" android:gravity="center" />
    </item>
</layer-list>

I don't know that solution was perfect or not, because I always implement it as a view and draw myself. But I think Android SDK supplied more convenient component let's finish work with xml configuration, we should be learn and use it.

You can use empty views to fill upp the space evenly between the radio buttons. In this case I have 3 radio buttons that vill look centered, nice and tidy.

<RadioGroup
    android:id="@+id/radioGroup1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal" >

    <View
        android:layout_width="0dip"
        android:layout_height="wrap_content"
        android:layout_weight="1" />

    <RadioButton
        android:id="@+id/radio0"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:gravity="center"
        android:text="" />

    <View
        android:layout_width="0dip"
        android:layout_height="wrap_content"
        android:layout_weight="1" />

    <RadioButton
        android:id="@+id/radio1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:checked="true"
        android:gravity="center"
        android:text="" />

    <View
        android:layout_width="0dip"
        android:layout_height="wrap_content"
        android:layout_weight="1" />

    <RadioButton
        android:id="@+id/radio2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:gravity="center"
        android:text="" />

    <View
        android:layout_width="0dip"
        android:layout_height="wrap_content"
        android:layout_weight="1" />
</RadioGroup>