Plant oil is needed for fuel source that can be recycled. However, plant oil still has shortage, which is high viscosity. Meanwhile, plant oil still has weakness, there is the high viscocity. Mix the plant oil in the biodiesel to fix the viscocity. Castor oil and palm oil are the potential plant oil to become the biodiesel fuel. The aim of this research is to know the impact of mixture composition towards the physical characteristic of biodiesel with castor oil and coconut palm oil for the fuel. For make a biodiesel, this research used castor oil and palm oil with transesterifikasi method also 15% of methanol from the oil volume. It used catalyst KOH (Calium Hidrochyde) with the comparison 1% with the oil volume. The mixture between Castor biodiesel and Palm biodiesel on the temperature 80 in 60 minutes, with the mixing composition between Castor biodiesel and Palm biodiesel 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 mixture Castor biodiesel and Palm biodiesel showed the decreasing graphic from the density test, viscosity and flash point, along with the increasing of coconut palm biodiesel on each variation mixture. On the graphic test of heating value experience the increasing on castor biodiesel with adding the palm biodiesel.