Abstarct: Bastam plain with an area of about 484 km2 is suited in Semnan province. The groundwater resources in the plain is of great importance particllarly in terms of drinking and agriculture uses. In order to optimize water resource management, it is necessary to identify the vulnerable areas to ground water contamination in a specific area. In this study, a GIS – baxsed Drastic model is used to evaluate the vulnerability of groundwater resource in Bastan plain. In this model, seven effective parameters including depth to water table (D), recharge (R), aquifer media (A), surface soil texture (S), topography (T), impact of unsaturated (I) and hydraulic conductivity(C) are considered. These are first rendered in GIS (as data laxyers) and then weighted, rated and overlaid to produce a vulnerability index. The overall Drastic index was found between 40 and 163 for the study area. According to zoning map, it was revealed that 17.56 %, 14.40 %, 34.40 %, 29.60% , 3.2 % of the total area have negligible, low, average, high and very high vulnerability, respectively. The vulnerability assessment also show that vulnerability in the central part, West regions and The northern part of the plain has high to very high vulnerability and other areas are characterized by moderate to low vulnerability. The high the vulnerability in these areas is evidently due to low slope, shallow ground water level and coarse texture of the aquifer and Soil. To validate the accuracy of drastic modelling nitrate concentration were measured in 14 abstraction well within in the Bastam plain. Nitrate concentration is varied from 9.6 to 80.6 milligrams per liter. Increasing trend in the central plain and its southern parts is obviously owing to extensive farming and the extensive use of nitrate fertilizers. overlapping the map of nitrate concentration with final produced map vulnerability was obtained.