A part of my models would inherits an abstract class

Salut !

I want re-use same code for a group of model. Is it possible to make some models inherits a class and how to do this ?

In advance, thank you.

Yes, make a base model and extend it.

models/Auth_model.php
class Auth_model extends MY_Model

core/MY_Model.php
class MY_Model extends CI_Model

Thank you, but can we make few models class in core/  ?

You can make as many you want.
But then I would place them in /models/ instead. As it's not going to be extended by all your models.