The major measure taken on global warming is to control greenhouse gas emission for different economic sectors. Within the context of the United Nations Framework Convention on Climate Change (UNFCCC), to which Turkey is party, parties are obliged to report annual greenhouse gas emissions from each economic sector. Greenhouse gas emissions from the waste sector account for approximately 3% of global human-induced greenhouse gas emissions, while wastewater treatment accounts for 20% of the greenhouse gas emissions from waste sector. In this study, factors effecting direct greenhouse gas emission in a wastewater treatment plant with an equivalent population of 2344000 were investigated by using GPS-X 6.5 software. Biological nutrient removal was conducted and sludge was digested under anaerobic conditions in the wastewater treatment plant. Both sludge age and dissolved oxygen parameters were investigated. 2, 5, 9, 13, and 18 days of sludge age were tested, and direct emissions of 0, 618508, 565961, 565681 and 554684 t CO2 eq/year were calculated, respectively. Direct emission amount decreased with increasing sludge age over five days. Dissolved oxygen emissions of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mg/L were tested, and direct emissions of 1455632, 999243, 719380, 583603, 503275 and 449997 t CO2 eq/year were calculated, respectively. Direct emission amount decreased with increasing dissolved oxygen concentration in the aeration tank. Considering sensitivity analysis, dissolved oxygen parameter is more effective on greenhouse gas emissions in comparison to the sludge age parameter.