The transesterification process using heterogeneous catalysts has been widely studied to replace the role of homogeneous catalysts. ZnO doping into metal oxides can increase activity of heterogeneous catalyst in transesterification reaction. This study was conducted to provide information on the effect of ZnO concentration doped into calcium oxide (CaO) to the transesterification reaction of avocado seed oil with high free fatty acid (ALB) to methyl ester, at 65 0C, methanol ratio: oil = 10: 1 , for 1.5 hours, using a reactor. Research variable is ZnO concentration doped into CaO, that is: 0%, 1%, 2%. The test parameters are methyl ester content obtained from the results of transesterification reaction with gas chromatographi analysis. In this study, ZnO/CaO nanocatalysts were synthesized and doped with sol gel method and calcined at 450 0C in air for 60 min. The synthesized ZnO/CaO nanoparticles were characterized by XRD. From the experiment, the highest yield of methyl ester was obtained on ZnO/CaO 1% catalyst with yield of 90,8820%.