17-4PH precipitation-hardening martensite stainless steel is an important commercially available stainless steel. And the mechanical properties of this steel are very sensitive to quenching and aging process. In this paper, the effects of aging process on mechanical properties of this steel were investigated. The results show that the microstructure of this steel consists of quenched martensite, tempered martensite and precipitates. With the increase of the aging temperature, the strength decreases and the toughness increase. And the fracture mode of this steel transforms brittle fracture into toughness fracture.