This paper presents comparative energy and exergy analyses of a No. 2 diesel fuel, marine fuel, and four various biodiesel fuels for a four-cylinder, four-stroke, direct-injection diesel engine. The engine was used to obtain the experimental data at full-load conditions for constant 1600 rpm speed. It was found that the first- and second-law efficiencies increased with biodiesel addition and the minimum exergy destruction was obtained at 100% biodiesel fuels. 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 diesel engines.