Abstarct: Shahrood plain with an area of about 540 Km2 is located in Semnan provience. In this research with the aim of to investigate the transport of nitrate, groundwater flow was firstly modeled by MDFLOW and then transport of nitrate was simulated using MT3DMS code. Groundwater flow model in steady state was calibrated baxsed on the average data for a period of 4 months from December 2007 to march 2008. Unsteady flow model was performed and calibrated for a period of 25 months from March 2008 to 2010. Values of hydraulic conductivity and Spesific yield was determind during the steady and unsteady calibration processes. Folw model was verified for a period of 12 months in 2012. Transport model of nitrate was developed from April 2010 to March 2011 considering initial and boundary conditions and transport mechanisms for nitrate (advection, dispersion, diffusion and chemical reactions). The model was calibrated baxsed on data in urban area from July 2003 to October 2005. Nitrate concentrations of groundwater vary from 5.5 to 140 mg/L in this area. Results showed that the nitrate concentration in urban area ranged from 5/5 mg/L to 140 mg/L. Regions with low hydraulic conductivity (1 m/d) at west and water table depression in the south of urban area large number of production wells prevented nitrate contaminant to spread in these areas. Also, the transport of nitrate is highly affected by groundwater flow and hydraulic conductivity of the aquifer. In adation, the results of nitrate transport model showed that nitrate concentration in recharge from urban area ranged from 70 mg/L to 450 mg/L and nitrate concentration in agriculture return flows is about 200mg/l. In the other word, the average flux of nitrate from agricultural areas was estimated to be at 152 Kg-N ha-1 year-1. Risk zone of nitrate pollution in urban area and nitrate attenuation after deletion of contaminant sources was also investigated in shahrood aquifer. Attenuation trends in urban area showed that maximum nitrate concentration is decreased to 114mg/L after one year and to less than 100mg/L after five years. Isu-concentration maps do not show important changes in agricultural areas during attenuation period. This is porobably related to the short time in model run and low volumes of return flow comparing the saturated zone of aquifer.