Masjed Soleyman reservoir is located in Zagros Mountain of western Iran, which is one of the most seismically active zones of the Alpe-Hymalaya belt. So, it seems to be necessary to carry out widespread studies, especially on the impact of this reservoir with 177 m height and 261 million m3 capacity on occurrence of induced seismicity in the surrounding region. The Gutenberg-Richter relation is one of the well-fitted empirical relations in seismology: it represents the frequency of occurrence of earthquakes as a function of magnitude: , where N is the cumulative number of earthquakes with magnitude larger than M and A and b are constants. In this paper we used b-value to study the heterogeneities in the crust beneath and around the Masjed Soleyman reservoir. In order to better understanding of the impact of this reservoir on seismic activity, a local seismic network of 5 seismological stations was installed in the area on June 2006. About 1924 Seismic events recorded during a period of 15 month were used in this study. We maped both surface and cross-section view of b-value in the region using the computer program ZMAP. The study area was divided into grids with spacing of 0.01o in latitude and longitude. A circle was drawn around each grid point and its radius was increased until it included N=50 earthquakes. The b-value was calculated by using a maximum likelihood method for the selected 50 earthquakes and the grid point was colored corresponding to the b-value. The results show high value of b-value due to reservoir induced earthquakes beneath the Masjed Soleyan lake. The most important factors known responsible for increased heterogeneity in this area, are reservoir loading and increased pore fluid pressure that cause occurrence of swarms and heterogeneous stresses in the area.