The excessive use of unrenewable energy can cause energy crisis problems. Vegetable oil as a renewable energy source can be used as an alternative fuel. Vegetable oil has a viscosity (thickness) 20 times higher than fossil diesel fuel, therefore it is necessary to react and convert it to methyl ester (biodiesel) with methanol and catalyst (KOH) through the transesterification process using a heating device. Vegetable oils that have the potential to be used as biodiesel are castor oil and palm oil. This research aims to determine the effect of temperature and reaction time of a mixture of castor oil biodiesel and palm oil biodiesel on the nature of biodiesel. Making biodiesel in this research using the transesterification method and using methanol 15% of the volume of oil. The catalyst used is Kalium Hydroxide (KOH) with a ratio of 1% to the volume of oil. Composition of the biodiesel mixture uses a ratio of 50:50 (%),with temperature and time variations of 60° C - 30 minutes, 60° C - 60 minutes, 60° C - 90 minutes, 90° C - 30 minutes, 90° C - 60 minutes, 90° C - 90 minutes, 120° C - 30 minutes, 120° C - 60 minutes, and 120° C - 90 minutes. The results showed that the nature of biodiesel was influenced by temperature and reaction time. Characteristic value of biodiesel will be higher as the temperature and mixing reaction time increase. Characteristics produced in this research are density, viscosity, flash point, and heating value. Variations of biodiesel mixtures that provide the most optimal properties are obtained in variations of 120° C - 90 minutes with a density value of 866,202 kg / m³, variation of 120° C - 60 minutes with a viscosity value of 6.178 cSt, variation of 60° C - 60 minutes with flash point value 175.93° C and variations of 120° C - 90 minutes with a heating value of 9167.0552 Cal / g.