Abstract In this study, performance and emissions of cottonseed oil methyl ester in a diesel engine was experimentally investigated. For the study, cottonseed oil methyl ester (CSOME) was added to diesel fuel, numbered D2, by volume of 5%(B5), 20%(B20), 50%(B50) and 75%(B75) as well as pure CSOME (B100). Fuels were tested in a single cylinder, direct injection, air cooled diesel engine. The effects of CSOME-diesel blends on engine performance and exhaust emissions were examined at various engine speeds and full loaded engine. The effect of B5, B20, B50, B75, B100 and D2 on the engine power, engine torque, bsfc's and exhaust gasses temperature were clarified by the performance tests. The influences of blends on CO, NO x, SO 2 and smoke opacity were investigated by emission tests. The experimental results showed that the use of the lower blends (B5) slightly increases the engine torque at medium and higher speeds in compression ignition engines. However, there were no significant differences in performance values of B5, B20 and diesel fuel. Also with the increase of the biodiesel in blends, the exhaust emissions were reduced. The experimental results showed that the lower contents of CSOME in the blends can partially be substituted for the diesel fuel without any modifications in diesel engines.