Temperature and wind fields over Tehran were simulated using the MM5 modeling system. To do this a nested domain with three nests with grid spacing of 27, 9 and 3 km were chosen. The model was run for two different days, one with a continental polar air mass (CPA) and the other one with a continental tropical air mass (CTA) over the region. The model outputs for the innermost nest were studied for the first 24 hours of the forecast. The model results were compared with observed 2m temperature and l0m wind speed at Mehrabad, Aghdasieh and Chitgar stations. The results show a fairly good agreement with the corresponding observations. The results also show that during the time of CPA, the Tehran heat island is well formed and has a relatively high intensity in the northern part of the city while during the time of CTA the heat island does not grow during the night time and has a lower intensity as well. Examining the model results also shows that the model is capable of capturing the micro and mesoscale characteristics features of the heat island and catabatic and anabatic winds over the city. Also, the results show that the mountain cooling intensities the urban heat island circulations at night and these circulations reduce heat island intensity.