The efficiency of existing municipal wastewater treatment plants has been affected by the increase of incoming organic load caused by the expansions in developing countries. In the present investigation, the effect of Chemically Enhanced Primary Treatment (CEPT) process was studied on the enhancement efficiency of a municipal wastewater treatment plant in Tehran, Iran. Jar test results showed an increase in COD, phosphorus, turbidity and TSS removal by the increase in dosage of aluminum sulfate and ferric chloride as coagulants. Results revealed COD, phosphorus, turbidity and TSS removals of 38, 66, 68 and 69 %, for alum at 80 mg/L and 60, 73, 49 and 48 % for ferric chloride at 70 mg/L as the optimum doses, respectively. Ferric chloride revealed more efficient results compared with alum. The result of tests to find the optimum pH of two coagulants revealed that pH of 8.2 for both chemicals is the optimum performance condition. Therefore, CEPT can be used as an efficient method in conventional municipal wastewater treatment plants to reduce the organic load of biological treatment and enhance the removal of nutrients.