The use of vegetable oil methyl esters has been proposed as an alternative fuel for diesel engines. The purpose of this study is to investigate the combustion of soybean oil methyl ester in a direct injection diesel engine, and compare it to that of a conventional diesel fuel. Experimental measurements of performance, emissions, and rate of heat release were performed as a function of engine load for different fuel injection timings, and injector orifice diameters.It was found that overall, the soybean oil methyl ester behaved comparably to diesel fuel in terms of performance and rate of heat release. The methyl ester fuel gave lower HC emissions and smoke number than diesel fuel at optimum operating conditions. The results for CO emissions were varied. NOx emissions were strongly related to the cylinder pressure development.Changing the injection orifice diameter had less effect on engine performance when using diesel fuel, than with methyl ester fuel. A smaller orifice diameter gave higher cylinder pressure and maximum rate of pressure increase, higher NOx emissions, and a larger amount of premixed burning for both fuels.The variation of injection timing had a pronounced effect on performance and emissions for both fuels.Conventional trends in emissions, performance and rate of heat release were observed for both fuels.