The world energy demand is increasing rapidly due to the excessive use of the fuels but because of limited reserves, the researchers are now looking for alternative fuels. Another serious problem associated with the use of petroleum fuel is the increase in pollutants emissions. In this study, cottonseed oil methyl ester was added to diesel fuel by volume of 5% (B5), 20% (B20), and 50% (B50). Blends were used in a single cylinder, direct injection, air-cooled diesel engine at partial and full load condition. In experiments, the effects of cottonseed oil methyl ester ester-diesel fuel blend on the engine fuel consumption and exhaust emissions were investigated at 2,000 rpm of engine speed that most engines run on. At the tests, the effect of blends on fuel consumption, exhaust gases temperature, CO, NOx, SO2, and smoke opacity was clarified. The experimental results showed that cottonseed oil methyl ester can be substituted for the diesel fuel without any modifications in diesel engines.