A process for the production of the ethyl ester of used frying oil for use as a biodiesel fuel has been studied. The essential part of the process is the transesterification of the used frying oil with ethanol, in the presence of a catalyst, to yield the ethyl ester as a product and glycerine as a by-product. Experiments have been performed to determine the optimum conditions for the preparation of the biodiesel. The optimum conditions were: (1) 0.5% sodium hydroxide catalyst (dissolved in ethanol) based on weight of used frying oil and 100% excess of the stoichiometric amount of required anhydrous alcohol; (2) Extremely vigorous agitation with a little splashing until the reaction mixture (oil and ethanol) becomes thoroughly mixed; (3) Microwave irradiation of the mixture at 50% of its exit power of 750 W. Viscosity, cloud point and pour point of the used frying oil ester produced were measured.