The solid base catalyst CaO was prepared and characterized by XRD technique and low temperature nitrogen adsorption-desorption technique respectively. Transesterification of castor oil to biodiesel using CaO as a solid base catalyst was studied. The separate effects of reaction temperature, molar ratio of methanol to oil, catalyst dosage and reaction time on methyl ester yield were investigated.Under the optimal conditions of reaction temperature 65 ℃, molar ratio of alcohol to oil 9∶1, mass fraction of catalyst 1.5% and reaction time 2.5 h, the methyl ester average yield is 92%. The product was analyzed by gas chromatographic technique and mass fraction of methyl ester is 97.6%.The products after reaction is dealt with easily and the environment is not polluted and the catalyst can be activated and reused.