Visas for US passport holders are subject to particular regulations. 
At present the Iranian Ministry of Foreign Affairs (MFA) does not allow US passport holders to travel to Iran independently.  Americans are required to travel on escorted tours; either as part of a tour group, or on a tailor-made individual tour. An exact itinerary, to which you must adhere, must be submitted in advance.

Due to these requirements we recommend that you make your travel arrangements at least 10 weeks in advance of your planned travel date. Please click here if you would like further information about guided tours to Iran.