In this work, the post-deactivation of irradiated catalase in dilute aqueous solution was found and investigated. Post-deactivation of irradiated catalase means that the catalase in dilute aqueous solution could not only be deactivated during γ-irradiation, but it has also been deactivated continously for some time after the irradiated samples were taken out of the radiation field. No reports about this phenmenon in literatures were searched up to now. The effects of absorbed dose,initial catalase concentration in solutions, atmosphere, temperature and additive on post-deactivation of catalase were investigated. H,2,O,2, produced by water radiolysis may attend the post-deactivation reaction in some way. Oxygen in ensyme samples is necessitous for the post-deactivation. 1×10,-4, to 5×10,-3, mol/L of CH,3,CH,2,OH, HCOONa and EDTA could control the post-deactivation efficiently.