Abstarct: South Mayamey Plain is located in Shahrood district at the east of Semnan province. High exploitation of groundwater has continuesly decreased quantity and quality of the aquifer. Therefore, hydrogeological assessment and prediction of the future situation of aquifer is needed for management. In this study, groundwater flow in South Mayamey aquifer was simulated by MODFLOW code in GMS program. Hydrogeological charactristics of aquifer were firstly evaluated baxsed on the results of pumping tests, log and hydrograph of observation wells and geophysical studies. Regarding to plain hydrograph, esfand 1388 was selected as the steady state priod. Conceptual model was prepared and converted to a numerical model. Model was calibrated for hydraulic conductivity (K) and inflows from boundaries. After that, model was calibrated for steady state and values of specific yield (Sy) and reacharge (from precipitation and irrigation return flow) were optimized, respectively. Verification of model was perfomed for one year after calibration period of Farvardin 1389 to Esfand 1389. Acording to calibration results, annual out flows from aquifer are more than inflow, confirming the declining trend of plain hydrograph. Average values of calibrated K and Sy were 8.5 m/day and 0.0064, respectively. Verified model was used to predict affects of different scenarios on Mayamey aquifer. At the first scenario, drought condition was simulated by 50 percent reduction in precipitation, representing water level in observation wells was significantly decreased. At the second scenario, precipitation was increased 50 percent for simulated of wet periods. Water level rised in observation wells in this scenario. At the third scenario, discharge by production wells was modified, showing loss drowdown in aquifer levels, which is not the same at different parts of aquifer.