Recently we have a growing interest in environmental pollution and alternative energy. Diesel engine is generally used to produce the power on shore and sea. However, the combustion characteristics and exhaust emissions of the engine are changed on account of the wear of fuel system and the altered ambient condition of the combustion chamber by the increment of the engine operation hour. Therefore the combustion characteristics and exhaust emissions on the fuel injection timing were experimentally investigated to find out the optimum fuel injection timing in case of the about 20 years used diesel engine using biodiesel blend oil. The original fuel injection timing of the engine is BTDC $22^{circ}$ CA. However, it is found that the optimum fuel injection as a result of analyzing the specific oil consumption and exhaust emissions of 20 years used the engine is BTDC $26^{circ}$ CA.