This research aims to evaluate the water quality of the Al-Sain lake which feeds mainly the Lattakia city and a part of the Tartous city of drinking water purpose. In this paper, we study a group of physical and chemical factors for the water of the Al-Sain lake during the years of available data (1991-2004-2007-2011), samples were taken from four different locations according to the program approved by the concerned staff to monitor and protect the lake, namely: (Lattakia water supply point, Tartous water supply point, irrigation point, southern spring observatory), and we calculated the quality index for drinking purposes using US approved methodology of National Sanitation Foundation. Nine factors have been adopted: (dissolved oxygen, fecal coliforms, pH, BOD5, temperature, phosphate, nitrates, turbidity, and total solids). The research result showed that the classification of water quality was good in all four monitoring points during the years 1991 and 2004, noting a significant improvement of the quality in 2004, and decreases slightly during the years 2007 and 2011 for monitoring points in the Lattakia water supply and irrigation points, but water quality still keep maintain a good classification, while the quality of the water goes down within the threshold of middle classification for the Tartous and southern spring observatory monitoring point.