i want to create my own function in wp-admin same like do in wp-content->functions.php, but i don't know where is the function file in wp-admin folder.
anybody know where is the function file in wp-admin folder?
There is no function file in "wp-admin" folder, you do not need to edit that folder.
Wordpress has Plugin api to do that, http://codex.wordpress.org/Plugin_API/
Create a plugin and write function in your plugin file.