Present research was conducted on comparative investigation of effluent quality of two water treatment plants, Bandar Gaz and Bandar Turkmen, in the east of Golestan province. Hence, the concentration of 11 qualitative parameters including temperature, pH, BOD, COD, EC, nitrate, TSS, TS, turbidity, total phosphorus, DO, endothermic coliform, and total coliform along with discharge as a quantitative parameter in effluent was investigated in 2022-2023. Results revealed that produced sewage volume has the least changes among measured parameters of both treatment plants. Results revealed that the amount of turbidity, total coliform and BOD of Bandar Gaz wastewater treatment plant is much higher than that of Bandar Turkmen. This indicates that in addition to physical pollution, the microbial pollution amount in effluent of Bandar Gaz treatment plant is higher than that of Turkmen treatment plant. BOD/COD ratio in entering effluent in both treatment plants is around 0.6. Therefore, the effluent of both treatment plants can be easily decomposed using biological methods. However, BOD/COD ratio in the output effluent is less than 0.3. Hence, there is no possibility of biological decomposition of wastewater. Removal rate results revealed that the highest removal rate is related to TSS and BOD, respectively. Results showed that the removal amount of parameters in Bandar Turkmen treatment plant is more than that of Bandar Gaz treatment plant, except for COD. ISQA water quality index revealed that an improvement of effluent water quality after wastewater treatment process. Although the effluent in both treatment plants is in a relatively weak class.