Abstract Background & objectives: Lakes and reservoirs have been noticed as the important resources to provide drinking and agriculture water, so the optimal usage of water resources demands suitable methods to monitor and define the quality of these resources. Regarding the importance of Ekbatan reservoir that provides main part of drinking water of Hamadan, so quality evaluation seems to be necessary. Material & Methods: This is a cross sectional study that sampling a in 12 months of 2010 in 7 different stations. Quality parameters that were measured including: pH, TS, BOD5, DO, Nitrate, Phosphate, Temperature, turbidity and fecal coliform. Data were analyzed with NSFWQI Results: according to NSFWQI index in studied stations during a year, the highest quality was seen in station No 1 (average quality: 65.24) and the lowest one was observed in station No 6 (average quality: 53.02). Based on the months, the highest quality was seen in February (average quality: 67.52) and the lowest one was seen in August (average quality: 52.76) Conclusion: lake water had higher quality in cold months of the year than those of warm months. In terms of the possibility of using lake water as drinking water, station No 1 and station No 2 had higher quality and with Conventional treatment it would be possible to use the water as drinking water.