The research mainaim is Mapping of Groundwater Vulnerability to Contamination Using (PI) Method toevaluatethe study area (Marqieh and Alhousen watershed ) in Tartous governorate. The area of study forms a part of the coast basin, on the east coast of the Mediterranean Sea, located north of the Tartous governorate between , 34 5000,35 1000latitude in the north of the equator, and , 35 5000,36 2000 longitude in the east of the Greenwicharea covers about 700 Km2. The PI method is a GIS-based approach(We use ARCGIS .10 and the tool : Spatial Analyst Tools) , and gives special methodological tools for karstic aquifer , geological conditions , climate , relief , land use , all these conditions are mapped separately as individual maps and then integrated to achieve the groundwater vulnerability map. all the maps related to the study area was created in digital form and apply analysis processes to create vulnerability map which is a map divided to the fields and every field represents the possibility of contamination of groundwater in this region, whether high or low . The results obtainedfrom this study indicate that about 41% of the study area is under high groundwater vulnerability, 59% is under moderate, low and very low vulnerability.