Biodiesel is one type of renewable alternative energy that has great potential to be developed. Biodiesel is a fuel consisting of a mixture of mono-alkyl esters of long-chain fatty acids made from renewable sources, such as vegetable oils or animal fats, one of which is Crude Palm Oil (CPO). Crude Palm Oil (CPO) contains free fatty acids in high levels, so treatment is needed to reduce free fatty acids by a reaction known as the esterification reaction. Then, the transesterification process is carried out to produce biodiesel (methyl ester). The purpose of this study was to analyze the effect of catalyst mass, a mole ratio of CPO to moles of methanol and the effect of adding THF co-solvent to biodiesel purity. The catalyst used is a heterogeneous catalyst from kapok fruit peel waste. Kapok fruit rind was calcined at 700˚C for 8 hours. The independent variable varied the mole ratio of oil to methanol in a 1:4 ratio; 1:6; 1:8; and 1:10 with a catalyst weight variation of 3 and 4%. Meanwhile, for the addition of co-solvent, variations of THF: methanol v/v 1:1 and 2:1, were carried out. The biodiesel properties such as density, viscosity, water content and acid number, were evaluated and compared with the Indonesian National Standard. The results showed that the transesterification reaction with the addition of co-solvent resulted in a higher methyl ester content than that without the addition of co-solvent. The highest yield of methyl ester without the addition of co-solvent was 79.16%, while the yield of the methyl ester with the addition of THF co-solvent with a ratio of 1:1 and 2:1 v/v to methanol was 90.09 and 94.09%, respectively. The highest methyl ester content (94.09%) was achieved by the addition of THF: methanol = 2:1, CPO: methanol molar ratio = 1:6 and 4%-wt catalyst weight. The results obtained in this study indicate that a green catalyst made from kapok skin can be used to produce biodiesel and also the addition of co-solvent can increase the yield of methyl esters, so that high purity is obtained.