How can I make space between two buttons in same div?

Put them inside btn-toolbar or some other container, not btn-group. btn-group joins them together. More info on Bootstrap documentation.

Edit: The original question was for Bootstrap 2.x, but the same is still valid for Bootstrap 3 and Bootstrap 4.

In Bootstrap 4 you will need to add appropriate margin to your groups using utility classes, such as mx-2.

Just put the buttons in a class ( class="btn-toolbar" ) as told by bro Miroslav Popovic.It works awesome.

<div class="btn-toolbar">
  <button type="button" id="btnSubmit" class="btn btn-primary btn-sm">Submit</button>
  <button type="button" id="btnCancel" class="btn btn-primary btn-sm">Cancel</button>
</div>

You can use spacing for Bootstrap and no need for any additional CSS. Just add the classes to your buttons. This is for version 4.