Potassium based heterogeneous catalysts supported in niobium and zirconium mixed oxide were used in transesterification reactions, with soybean oil and ethanol, in order to produce biodiesel. First, the support was prepared by the impregnation of the hydrated zirconia and a peroxoniobium solution for 24h under stirring. Then it was dried and calcined, that led the formation of the mixed oxide Zr6Nb2O17. The support was impregnated by incipient method with a KOH solution, and then it was dried and calcined. The catalysts were employed in the transesterification reaction using a factorial design of experiments, 24, and the four variables were: KOH contents, amount of catalyst in reaction, ethanol:oil molar ratio, and time of reaction. Conversions more than 77% were obtained in all of experiments. The variables with more significance were the molar ratio and the amount of catalyst, and in all experiments with the high level of both were obtained conversions more than 99%.