Biodiesel is an alternative fuel for diesel engines made from vegetable oils or animal fats. However, the raw material of vegetable oil still has weaknesses in its characteristics one of them is high viskositas. To improve the characteristics of vegetable oil one of them is by mixing raw material of vegetable oil. Vegetable oils that have potential to be used as biodiesel are castor oil ( Jatropha Curcas ) and corn oil ( Corn Oil ). This study aims to determine the effect of mixing composition on biodiesel characteristics and obtain the optimal mixture. The process of making biodiesel in this research is using two steps. The first step is esterified with methanol and catalytic reactants homogeneous acid (H2SO4) with a reaction time of 60 minutes and a temperature of 60 °C. The second step is transesterification with methanol bonding and homogenous base catalyst (KOH) with reaction time 60 minutes and temperature 60 oC. Furthermore biodiesel of oil mixture with various variation of predetermined composition will be tested characteristic that is density, viscosity, flash point and calorific value . From the research data, it can be concluded that mixed biodiesel characteristics in each variation of composition tended to decrease in density, viscosity and flash point value along with the increasing of corn oil composition. But inversely proportional to the value of calories that have increased . the composition of the most optimal mix of biodiesel are jatropha curcas oil 40% and 60% corn oil with a density value of 857,6 kg/m3, a viscosity of 7.9 cSt, a flash point of 188 °C and 9244.77 calorific value Cal /g.