A process for preparing a dialdehyde which comprises subjecting a glycol to oxidation-dehydrogenation in the presence of silver particles of which the surface is partially coated with a silicon carbide powder or a silicon nitride powder. According to the present invention, dialdehydes can be prepared from glycols in high conversion of the glycol in high selectivity into the dialdehyde, and the silver catalyst can be reused even if the reaction is stopped in a while since the silver particles are not aggregated during long time operation.
