Abstarct: Shahrood plain with an area of about 540 km2 is located in Semnan provience. According to importance of groundwater resources in this plain which are used for drinking, agriculture and industry the study of the aquifer vulnerability and identify areas susceptible to contamination is necessary for development and management of valuable water resource. In this research vulnurability of Shahrood plain aquifer was evaluated using DRASTIC. Data laxyer including depth to water table (D), recharge (R), aquifer media (A), surface soil texture (S), topography (T), impact of unsaturated zone (I) and hydraulic conductivity (C), was prepared in GIS and wheighted. The laxyers were overlaid and DRASTIC index for the aquifer zones of Shahrood plain was calculated. It was estimated at 44 to 169. According to the aquifer vulnerability zoning map, urban areas and southern parts of the region are in high vulnerability class, and other areas are in moderate to low vulnerability classes. The fine texture of surface soils in agricultural areas is caused the low vulnerability of this region. Nitrate concentration in different parts of the shahrood aquifer with was compared the vulnerability zoning map. The results showed that nitrate concentrations is high in the vulnerable zones. In order to perform sensitivity analysis of the aquifer vulnerability maps with respect to different parameters, the correlation cofficients were investigated. Sensitivity analysis using methods of removing parameter and single parameter showed high sensitivity of vulnerability index to aquifer media and impact of vadoz zone laxyers. It proves more importance of these two laxyers in the aquifer vulnerability zoning.