The energy needs of fossil fuels over time have increased rapidly. Fossil fuels can’t be renewable, so utilizing other potential alternative fuels becomes one solution to reduce the consumption of fossil fuels. The alternative fuel currently being developed is bio fuel. However, this energy source has some drawbacks such as high viscosity and low calor value. The mixture of bio fuel is one of the things to improve the characteristics of its fatty acids. Raw materials of bio fuels that can be potentially used for fuel include soybean oil and castor oil. This research on the mixture of soybean oil and castor oil was done by varying the composition and the mixing time. The comparisons of mixed variation composition were 100: 0, 90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80, 10:90 and 0:100 ( %). The duration of heating the mixture was done for 30 minutes and additional 60 to 90 minutes for special mixture -50:50 (%) to observe the variation of time. The findings of the research show that the mixture of soybean oil and castor oil with various composition variations and mixing time variation along with its heating affects the characteristics of fatty acids. More composition of the castor oil mixture in soybean oil can affect the fatty acid content, so the density value, viscosity, flash point increase and the calor value decreases. The longer mixing time and heating can affect the fatty acid content and trigger the occurrence of oxidation causing the density, viscosity, flash point to increase and the caloric value to decrease.