Background and Objective: Water quality is one of the issues that is of great importance to economic and social development due to the high costs of providing healthy drinking water and its risk to human health and the environment. For instance, although only a short time has passed since the construction of the Shirin-darreh dam, the problems arising from its exploitation have challenged the environmental sector and the health of the water. Materials and Methods: Water quality is affected by complex processes and various variables. Accordingly, NSFWQI input variables at monthly intervals collected at the dam reservoir during December 2011 - November 2012 was used to evaluate the water quality of the reservoir based on the grey relational analysis process. The results were compared with the results of the NSFWQI method. Also, the Shannon Entropy method was used, considering the importance of weight of variables in different methods. Results: The results showed that the water quality in the grey analysis method from November 2011 to May 2012 and in the NSFWQI method from October 2011 to April 2012 was moderate and in the remaining months was bad. In the grey analysis method, the highest reservoir value was related to Station 3 and month of February 2012 with a value of 64.2 and the worst water quality was related to Station 4 and month of August 2012 with a value of 39.2. Also, In NSFWQI method, the highest reservoir value was related to station 3 and month of February 2012 with a value of 68.7, and the worst water quality was related to Station 1 and month of August 2012 with a value of 39.1. Conclusion: Although there was no significant difference between the two methods, due to the greater adaptability of the results of the gray analysis method with the weather events and upstream conditions of the dam, this method is recommended. Due to the relative inadequacy of Shirin-darreh reservoir water in some months, it is essential that the source of pollutants from upstream areas is identified. After identification of the hotspots and preparation of the critical areas map, the appropriate control measures could be implemented.