The growth of Indonesia's population for each year is always increasing. With the increasing population of Indonesia, the need for fuel oil (BBM) also increased. The problem of global energy requirement in Indonesia, it needs alternative energy as a substitute for fossil fuel. One of alternative energy to replace fossil fuels is biodiesel. Biodiesel is an alternative fuel obtained from vegetable oil composed of methyl esters obtained through the transesterification process of triglycerides or the esterification of fatty acids. However, the vegetable oil still has weaknesses, one of which is high viscosity. To improve the viscosity one of them is by mixing vegetable oil with other vegetable oil types. This study aims to determine the effect of variations in composition of castor oil mixture and used cooking oil on the characteristics of biodiesel. The study the method used to make biodiesel was the esterification with a homogeneous acid catalyst (H2SO4) and the transesterification process was used a homogeneous acid base catalyst (KOH), with a reaction time of 60 minutes at 90⁰C. Variations in composition between castor oil and used cooking oil are 100: 0, 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, 10:90, 0: 100 (%). The result of the research data can be concluded that the density, viscosity and flash point tends to decrease with increasing composition of used cooking oil mixture, but with the raise composition of used cooking oil the yield of caloric value tends to increase. The most optimal mixture of the biodiesel mixture was obtained in the composition of Mj40Mg60 and Mj20Mg80, because the composition the viscosity value was the closest to the SNI 7182-2015 standard while the density and flash point values was in accordance with SNI 7182-2015.