In this study, the potential possibility of biodiesel fuel was investigated as an alternative fuel for a naturally aspirated common rail diesel engine. The smoke emission of biodiesel fuel 30 vol%(max. content) was reduced in comparison with diesel fuel, that is, it was reduced approximately 60% at 4000rpm, full load. But, power, torque and brake specific energy consumption didn't have no large differences. But, NOx emission of biodiesel fuel was increased compared with commercial diesel fuel.