Biological and ecological studies of water resources play an important role in both researches and scientific studies. The most important step for this study is identifying the characteristics of the ecosystem, living organisms and environmental factors. In recent years, Water quality evaluation, especially drinkable water, became both important and essential for reduction in quality and quantity of them. Lakes and ponds always being under attention as the most important resources for drinking and agricultural purposes. To improve efficiency of uses of the water resources, we need appropriate methods to determine quality of them. Investigate the quality of the Zayandeh roud lake water as an important resource to supplying drinking and agricultural water, in its basin and other province is necessary. In this study after determining 5 stations in different parts of the lake, sampling was done one time per every 45 days. WQI index calculated with use of 6 parameters: No₂, No₃, DO, EC, Hardness and, pH. However, despite of decreasing of the water quality of zayandeh roud lake, but according to WQI index, water of the lake was in the good category (50-100). This study was also showed that the water quality of zayandeh roud lake need a comprehensive management.