The need for conventional fuel is increasing. This encourages the development of renewable fuels that can reduce the adverse effects of the combustion process. One substitute for conventional oil fuel is vegetable oil. Biodiesel fuel from vegetable oils in addition to renewable fuels does not have a negative impact on the environment. But fuel from vegetable oils still has a disadvantage when used as fuel, the viscosity in vegetable oils is much higher than diesel. In this study, researchers conducted experiments to determine the effect of certain treatments on other controlled conditions. And to obtain the optimum concentration of CaO catalyst percentage of 5%, 10% and 15% in the reaction of making castor oil biodiesel by transesterification. The results obtained in this study are the characteristics of castor oil indicating that the amount of CaO catalyst mixture can reduce the characteristic value of castor oil during the transesterification process.