The usual transesterification of waste frying oil with ethanol does not give final concentration of biodiesel above 96.5%. The aim of this paper is to optimize the reaction conditions using ethanedioic acid as an activator of the carbonyl glyceryl ester. With the aid of a factorial design, the effects of the molar ratio of ethanol and oil, as well as the concentration of the catalyst sodium hydroxide were evaluated. The oil used was dehydrated and filtered, and the reactants submitted to mechanical stirring for two hours at 70° C. The data have been modeled by applying the response surface methodology, for the optimal reaction conditions. As results, the presence of ethanedioic acid drives transesterification from waste frying oil giving biodiesel with ester content over 96.5%. The response surface methodology gave the optimum conditions for the production of ethyl esters at molar ratio ethanol:oil of 6:1 and sodium hydroxide content of 1% relative to the mass of oil.