Abstract: In this paper the effect of long-term aging on static tensile properties of 17-4 precipita-tion-hardening stainless steel was studied,and the fracture morphology and the microstructure evolution was observed mainly by scanning electron microscope（SEM） and transmission electron microscope（TEM）,re-spectively.The results showed that with the age time prolonging,yield strength and tensile strength increased,but contraction of area and elongation was decreased.Smaller ductile dimples on the fracture of the quenched and tempered steel became into lager ductile dimples on the fracture of the long-term aged steel,and there was secondary cracks on the fracture of the long-term aged steel.TEM observation showed that microstruc-ture of the steel subjected to long-term aging took place spinodal decomposition,transforming to Cr-rich α’ phase and Fe-rich α phase,which was principally responsible for strength increasing and ductility decreasing.