Abstarct: Recharge estimation is one of the most important issues in groundwater resource management. The amount of recharge to the alluvial aquifer of Semnan with an area of 543 square kilometers has been investigated in this study. This aquifer is the main source of drinking water and agriculture in this area. Knowing this main source can solve management problems. In order to calculate the amount of recharge, four methods of water level fluctuations, chloride mass balance, stable isotopes and water balance have been used. Examination of the hydrograph of Semnan aquifer shows that this aquifer has an average decline rak of about 80 cm per year. According to the calculations, the amount of feed to the Semnan aquifer baxsed on the method of water level fluctuations equal to 4.4 million cubic meters per year, chloride mass balance method 2.5 million cubic meters per year and water balance method equal to 7.6 million cubic meters per year. The results of isotopic analysis show that the effect of evapotranspiration in the recharge process is not detectable, which confirms the insignificant role of surface recharge on the aquifer. According to the combination of the results of recharge calculations in different ways, the amount of rainfall to the Semnan aquifer was estimated at about 2 to 4 million cubic meters per year (average 3 million cubic meters per year), which is about 5% of annual rainfall.