Abstract: The strength and toughness of over-heated HAz of 35CrMo steel can be improved after double-phase zone(DPZ)trcatment.The toughness of over-heated HAZ is optimal when quenching temperature corresponds to the high temperature zone of DPZ. In this papcr,the mechanism of strcngth and toughness improvement is discusscd in three aspects,i.e.,microstructure of ovcr-heated HAZ, morphology of undissolved ferrite and temper brittleness.