Earthquake size distribution follows a power law whose slope is known as the b-value and its constant is named as the a-value. The b-value fluctuations have been theoretically studied in laboratories and practically investigated in several seismotectonics zones e.g. volcanic areas, continental rifts and mines which also present different stress regimes. The b-value can explain the relative density of large and small events which has found many applications in seismic hazard studies, spatio-temporal prediction and earthquake physics. On the other hand the a-value is concerned with regional seismicity level, so studying these parameters can be of great help in an area just like Tehran where there is a high concentration of people and social-economical activities. To begin this study we extracted the events from the Tehran Digital Seismic Network database. Processing followed by removing time-dependent quakes under the examination of the Poissionian assumption and later by computing the magnitude of completeness using the goodness of fit method. Then a-value and b-value changes were mapped in time and space. The b-value temporal changes are not significant during the period of data which may be under the control of local effects but reduction by depth exists. Both values show a change of around 51.5E. Meanwhile the b-value map shows a reduction toward regions with high density of thrust and strike-slip faults.