The methyl esters of soybean oil, known as biodiesel, are receiving increasing attention as renewable fuels for diesel engines. Biodiesel has a high cetane number, and offers the potential of emission reduction. The properties of biodiesel vary depending on its composition, and this may affect engine performance and emissions. In this project, biodiesel fuels were prepared from feedstocks with modified compositions including the methyl esters of a low palmitic soybean oil, a partially transesterified soybean oil, a synthetic blend of saturated esters, and a commonly used methyl soyate. These esters were blended with No. 2 diesel fuel in 20% and 50% concentrations. The blended fuels were then tested in a diesel engine to investigate the effect of biodiesel composition on performance, combustion characteristics, and emissions. The results show that the engine performance and combustion process of all the blends were similar to No. 2 diesel fuel with a slightly higher fuel consumption, a shorter ignition delay, and a lower premixed burning rate, with the exception of the blend of 20% partially transesterified fuel. All of the blends except the 20% partially transterified soybean oil demonstrated reductions in CO, HC, total particulate, and soot. The NOx emissions increased for all of the fuel blends. The blends with saturated esters provided a faster burning rate and a better emission reduction than the other fuels. These blends also showed the smallest increase in NOx emissions among the fuels tested.