Effects of high temperature calcination of ion-exchanged H-ZSM-5 (Me/H-ZSM-5) and metallosilicates having MFI structure in air with and without steam on the BET-surface area and NO conversion were examined. Metallosilicates were much more stable than Me/H-ZSM 5 in both BET-surface area and catalytic performance of NO conversion in the presence of excess O-2 and a low concentration of hydrocarbons. Furthermore, in case of H-Co-silicate, catalytic properties of both hydrocarbon combustion and NO conversion were significantly increased by a high temperature calcination in air stream including steam. The reason is that Co incorporated in the framework was partly but uniformly educed by the calcination, especially with co-existence of steam, and exhibited higher catalytic activity.