Performance parameters and exhaust emissions of a diesel engine fuelled with diesel fuel and a biodiesel, namely cottonseed oil methyl ester (COME), subjected to preheating at different temperatures in order to lower its viscosity have been investigated. COME was prepared using cottonseed oil, methyl alcohol and potassium hydroxide as a catalyst. Tests were carried Out at full load conditions in a one-cylinder, four-stroke, direct injection diesel engine. Before supplied to the engine, COME was preheated to four different temperatures, namely 30, 60, 90 and 120 degrees C. The test data were used for evaluating the brake power and brake thermal efficiency (BTE) together with CO and NOx emissions. The results revealed that preheating COME tip to 90 degrees C leads to favourable effects oil the BTE and CO emissions but causes higher NOx emissions. Moreover, the brake power increases slightly with the preheating temperature up to 90 degrees C. When the COME is preheated to 120 degrees C, a considerable decrease in the brake power was observed due to the excessive fuel leakage caused by decreased fuel viscosity. The results suggest that COME preheated tip to 90 degrees C can be used as a substitute for diesel fuel without any significant modification in expense of increased NOx emissions. (C) 2008 Elsevier Ltd. All rights reserved.