Abstarct: This study investigates the influence of the sewage project on the groundwater surface in the shahrood plain using Ground water modeling. Groundwater flow models are generally used as appropriate tools to predict aquifer response under the influence of recharge and discharge conditions. Sewage wells are the major sources of recharge in Shahrood Plain. It is expected that the performance of the sewage collection scheme in the Shahrood city will considerably affect on the quality of groundwater. A numerical finite difference model using GMS6.0 commercial software has been presented to simulate the effect of the sewage collection scheme on Shahrood aquifer. The numerical modeling was performed for a 1-year period (2008-2009). The simulation was first performed under a steady state condition for a time period of 1-month (January 2008) in order to calibrate the model. The model was then run under a transient case (from 2008 to 2009) to optimize specific yield, recharge and discharge values. The model was finally verified for a 1-year hydrological data (2009-2010). A close agreement was achieved between model predictions and field data. In order to consider the effect of sewage system on groundwater surface, the recharge from the sewage wells were removed from the total recharge and the model was again performed for a 10-year period. The results obtained from the numerical model show that the groundwater surface is dropped significantly in most of observation wells if the sewage system is performed. The drawdown can be reduced by amending water supply wells in the study area.