After the work done, I just cant find the option of "rescanning faces" !
How thankful as I saw the tips of enable rescanning in apple-support-discussions by BobbrkerMyHero.
Here comes the solution of rescanning faces.
**Before we start I recommend you BACKUP your iphoto with time machine or manually, or whatever you want to do. If you don't back it up and for some reason it doesn't work, don't blame me**
If you want to rescan the faces in your library this is all you need to do:
***CLOSE IPHOTO BEFORE YOU START THIS***
Open finder, go to your pictures folder.
-Find the "iPhoto Library"
-Right click (control click) on it
-Click "Show Package Contents"
-Find the files that hold the face detection information
Should be something like:
"face_blob.db"
"face.db"
-You need to move these files out of the folder, you can either delete them, or just move them to the desktop just in case something goes wrong.
-Open iPhoto back up, and it should rescan for faces!
-If everything goes correctly, you can delete the old files you moved to your desktop if you didn't delete them before.
