Abstract: The paper analyzed the components of the castor oil biodiesel by gas chromatography-mass spectroscopy. Comparative tests were made on a diesel engine fueled by the blended fuel of castor oil biodiesel and diesel in terms of the fuel consumption and emission characteristics. The results show that the basis of castor oil biodiesel is ricinoleic methyl ester. The rate of ester exchange is over 96%, and the physico-chemical properties of the castor oil biodiesel are almost the same to those of mineral oil. The function of diesel engine using the admixture fuel was improved as follows: the power is a little lower, the fuel consumption is a little higher, but the emissions of HC and CO are decreased, and the emission of NO_X is increased, the emission of smoke is decreased obviously.