This article investigates the influence of cetane number improver on heat release rate and emissions of a high-speed diesel engine fueled with ethanol-diesel blend fuel. Different percentages of cetane number enhancer (0, 0.2, 0.4%) were added to blends, and the engine tests were performed on a 4-cylinder high-speed DI diesel engine. The results show that: the brake specific fuel consumption (BSFC) increased, the diesel equivalent BSFC decreased, the thermal efficiency improved remarkably, and NOx and smoke emissions decreased simultaneously when diesel engine fueled with ethanol-diesel blend fuels; NOx and smoke emissions further reduced when CN improver was added to blends. From the combustion analysis, it can be found that the ignition delay prolonged, and the total combustion duration shortened for ethanol-diesel blend fuels when compared to diesel fuel; the combustion characteristics of ethanol-diesel blend fuel at large load may be resumed to diesel fuel by CN improver, but a large difference exists at lower load yet. (C) 2004 Elsevier Ltd. All rights reserved.