Abstract: Al₂O₃ supported KNO₃ (KNO₃/Al₂O₃) catalyst was prepared and characterized by XRD, DRIFT, nitrogen adsorption, ICP and basicity titration. The catalytic performance of KNO₃/Al₂O₃ in the transesterification of soybean oil with methanol for biodiesel production was investigated; especially, the nature of active species and deactivation behavior during the reaction was considered. The results illustrate that KNO₃ on the surface of Al₂O₃ is completely decomposed after calcination at 873 K, forming potassium aluminate. During the transesterification reaction, aluminate as the active species in KNO₃/Al₂O₃ is gradually dissolved out, which is responsible for the high catalytic activity of KNO₃/Al₂O₃. The biodiesel and glycerol products formed have a great influence on the catalyst activity and the catalyst deactivation is mainly ascribed to the saponification of biodiesel with the active species.