My Visa request for Schengen Visa has been refused by German embassy in Tehran. I've got the refusal notice explaining the reasons for the refusal. But I don't find any stamp on my passport. Is that normal or they've forgot it?
It's not necessary to have a refusal stamp in your passport.
The fact of your visa refusal will be recorded in the Schengen Information System, and the information will be available to the consulates and border officers of all Schengen countries. The same reasons for refusal that are ticked on your refusal notice are also recorded.
Thus, all Schengen states will be aware that you have had a previous visa refusal from Germany, and the reasons for the refusal that the consulate gave to you.