Biodiesel produced by the transesterification of vegetable oils (VOs) has recently become more attractive on account of its environmental and economical benefits. In this work, a heterogeneous catalyst system was developed for the production of biodiesel from used VOs using a continuous process. The free fatty acids (FFA) contained in the used VOs, which cause several severe problems in transesterification catalysis, were converted to fatty acid methyl esters (FAME) before the main biodiesel production process. The activities of several heterogeneous catalysts on the conversion of FFA were tested, with a WO3/ZrO2 Catalyst finally being selected. A method for preparing pellet-type catalysts was also developed. The pellet-type WO3/ZrO2 catalyst showed highly active and durable catalytic activities in the continuous flow process. The steady state conversion of ca. 70% was obtained in a 140 h durability test. The acidic property and catalytic activity of WO3/ZrO2 were attributed to the oxidation state of tungsten. (c) 2007 Elsevier B.V. All rights reserved.