This paper presents a comparative energy and exergy analyses of a six-cylinder, four-stroke, direct injection diesel engine using diesel fuel and its mixture with rapeseed oil. Engine was used to obtain the experimental data at full-load conditions for constant 2100 rpm speed. It was found that the first-and second-law efficiencies were increased with biodiesel addition. On the other hand, exhaust exergy was found to be inversely proportional to the biodiesel addition. It is also shown with extensive emission analysis that the addition of biodiesel fuels to the standard diesel fuel enhances the emission characteristics of our diesel engine.