But when i config file the db.php file in config/development folder it not working. And the project used to the db.php file in config folder. I don't understand why?
I have declare ENV in the bootstrap file as below:
Remember that the config/db.php and config/<your-environment/db.php are merged to create a single config file.
So you should only put global defaults in config/db.php, and nothing else. if you say it uses info from db.php, perhaps you've put a database config in there (which you should not do)?
That doesn't help much, since you've cut out all relevant information.
In your controller, do: \Config::load('db', true); var_dump(\Config::get('db'));
and post the result on http://bin.fuelphp.com. Add both config/db.php and config/development.php too, and post that link here (will be a lot easier to read). (replace username and password by *****)