Background & Objectives: Selection of the appropriate process is one of the most important issues prior to design and construct any wastewater treatment plant. Considering the importance of this issue, current study was carried out to assess wastewater treatment plants in operation in Ardebil, Tabriz, and Uremia in order to prioritize and select the most appropriate treatment process. Methods: Current descriptive-analytical study was conducted in 3 wastewater treatment plants of Ardebil, Tabriz, and Uremia in 2012 for a period of one year. Data were collected through a researcher-designed questionnaire. 30 participants were selected using census method. Data were analyzed by "Expert Choice" software. Results: Biolak process with a weight of 0.421 was selected as an optimal treatment process and Activated Sludge with a weight of 0.356 and Aerated Lagoon process with 0.223 held next priorities. Within the criteria, technical criterion with a weight of 0.407 was determined as the most effective criterion and environmental and economical criteria took next priorities. Conclusion: Considering importance of the selection of municipal wastewater treatment process to allocate funding reasonably and to achieve desired results, it is necessary to evaluate processes based on economic and engineering criteria prior to choosing an appropriate process.