The increasingly thin supply of fuel and the increasing price of fuel oil (BBM) for that need to be done to develop alternative fuels. Biodiesel is one alternative fuel, biodiesel is a mixture of methyl esters with fatty acids, liquid fuels that are processed from animal fat or vegetable oil, vegetable oil is a promising source of raw materials for biodiesel production because it is renewable and can be produced on a large scale. This study aims to determine the effect of variations in the composition of the mixture of biodiesel on the characteristics of biodiesel and mixed compositions that provide the most optimal properties. Biodiesel castor oil and palm oil biodiesel are made through a transesterification process using Homogeneous Base Catalyst (KOH), with reaction time of 60 minutes and temperature of 65⁰C. After transesterification process both of types of biodiesel then mixed with mixing variation 100: 0, 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, 10:90 , 0: 100 (%). Then the result of mixed variation done density test, viscosity, flash point and calorific value. The results of this study can be concluded that the value of density, viscosity and flash point tends to decrease along with the increase in the composition of palm oil biodiesel, but the calorific value increases with increasing composition of palm oil biodiesel. The composition of the mixture of castor oil biodiesel and palm oil biodiesel which provides optimal properties is obtained in the composition of 50% castor oil mixture: 50% palm oil. The characteristics produced include density value of 859.27 kg/𝑚3, viscosity of 5.8 cSt, flash point of 181.33 ⁰C, and the resulting heating value of 9013.33 cal / g. With the characteristic values produced, all the test parameters meet SNI 1782-2015 standards.