Background and Objective: Considering importance of wastewater treatment and disposal in contemporary societies as well as the possibility of effluent discharge of Wastewater Treatment Plants (WWTP) to water courses, or reusing it this study conducted to assess efficiency of Zanjan WWTP in 2008. Methods: This cross-sectional study was performed from June to March 2008 in Zanjan WWTP. Sampling was accomplished from inlet and outlet flow of treatment plant and measurement of TSS, BOD5,COD, MLSS, MLVSS, and SVI indices in aeration ponds were done based on standard method version 20. Results: Removal efficiency of TSS, BOD5, and COD was 77.91, 87.25 and 87.29 percent with respective average concentration of 30.2, 18.63 and 33.37 milligram per liter in outlet flow. These results are in accordance with standards of Iranian Department of Environment to be used for agricultural purposes. Conclusion: efficiency of pollutants removal in Zanjan WWTP is around 84 percent which complies with Iranian regulation, thus the flow is suitable for reuse or discharge to water courses.