In this study, the combustion, performance and emission characteristics of conventional diesel fuel and biodiesel produced from soybean oil and its blends (B10, B20, B50) were compared. The tests were performed at steady-state conditions in a single-cylinder direct injection diesel engine over the entire rpm range (1200-3000 rpm). During the tests, the fuel consumption, pollutant emissions, exhaust temperature and in-cylinder pressures were measured. The experimental results, showed that, relative to diesel, biodiesel had a 1-4% decrease in the torque and an approximately 2-9% increase in the brake-specific fuel consumption (BSFC) due to the lower heating value (LHV) of the biodiesel. However, biodiesel significantly reduced carbon monoxide (CO) (28-46%) and unburned total hydrocarbons (THCs), while the nitric oxides (NOx) (6.95-17.62%) and carbon dioxide (CO2) emissions increased slightly 1.46-5.03%. The combustion analyses showed that the addition of biodiesel to conventional diesel fuel decreased the ignition delay and reduced the premixed peak. These results indicated that biodiesel could be used without any engine modifications as an alternative and environmentally friendly fuel. (C) 2012 Elsevier Ltd. All rights reserved.