How can I hide the Tool-bar in Emacs persistently?

Add the following to your init file (~/.emacs or _emacs or ~/.emacs.d/init.el):

(tool-bar-mode -1)

Emacs has a nice built-in customization interface.

Select Options › Customize Emacs › Specific Option, start typing tool, then hit TAB to see the options starting with tool. Choose tool-bar-mode then. Toggle its value to switch it off, and press Save for future sessions.

I agree with michael. But if you only add this line to your .emacs file, there will be errors when you run emacs in the command line mode. Thus, a better solution may be adding the following to you .emacs file:

(if window-system
    (tool-bar-mode -1)

so that, tool bar will be hidden only when you run it in GUI. Emacs in command line mode does not seem to have a tool bar.