PW) were prepared and used as acid catalysts for transesterification and esterification reactions. These catalysts have double acidity properties, i.e., Lewis acidity and Brønsted acidity, that are suitable for the conversion of waste cooking oil into biodiesel. The highest efficiency was 59.2 % and 94.7 % corresponding to transesterification and esterification reactions by Ti 0.6 H 0.6 PW with moderate Lewis acidity. The relationship between the acidic properties and the catalytic activity is discussed in detail.