Complex topography in the North West of Iran (NWI) affects the formation and distribution of various precipitation mechanisms. In this study, regional climate model (RegCM4) is used to evaluate the role of mountains and their thermal and mechanical forcing in distribution of spring precipitation. For this purpose, two experiments with the topographical features and with the smoothing mountain ranges of the region are conducted. The results showed that by smoothing the topography, in addition to decreasing the significant share of precipitation in the interior NWI, the regional pattern of the amounts of diabatic heating in particular the horizontal advection of heat, which plays an important role in compared the rest of terms, also were changed fundamentally. In fact, with the removing of topography, the thermal contrasts from the mountains significantly reduced; also, the spatial distribution and the amount of spring precipitation affected. Therefore, it can be say that the mountains along with large-scale synoptic forcing will play an important role in the heating and mechanical mechanisms of this region precipitation event.