Abstract: 17-4PH stainless steel was tempered at 540 ℃ for different time after solution treatment at 1 040 ℃ for 1 h and quenched. The precipitation strengthening effects were characterized by hardness testing and the microstructure evolution of precipitates was characterized by atom probe tomography (APT). Double hardness peaks occur on the hardness curve. The APT result shows that the first hardness peak is caused by the precipitation of a large number of nano-sized Cu-rich phases, and the second hardness peak is caused by the precipitation of Cr, V, Nb-rich carbonitride. The APT result also shows that Mn, Ni and Al elements segregate at the interface between Cu-rich phase and matrix.