I think I found the answer . I forgot to use createrepo in order to create thex ml-based rpm metadata.
Then , the complete answer would be :
# vim netinstall.repo
PS:[root@localhost yum.repos.d]# mount|grep netinstall
/ISOs/CentOS-5.10-x86_64-netinstall.iso on /media/net type iso9660 (rw,loop=/dev/loop0)
As you can see , /ISOs/CentOS-5.10-x86_64-netinstall.iso is mounted on /media/net ,and I created the repo in /media and the baseurl is ///media .
I hope i didn't forget anything .
I think it is okay .
Sorry , I 've just started learning Linux .