The microstructure and mechanical properties of as-deposited and heat-treated laser solid forming (LSF) 17-4 PH (0Cr17Ni4Cu4Nb) percipitation-hardening stainless steel are investigated. The results of optic microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD) show that the microstructure of as-deposited LSF 17-4 PH steel consists of lath martensite and a small quatity of the second phase strengthening particles which distribute in and between the laths. According to the characteristics of alloy, the precipitating second phase strengthening particles should be M7C3 and NbC carbides, etc. The microstructure at the bottom of the deposited LSF 17-4 PH is mainly thin lath hardened martensite, however, there are thick lath martensite at the top of the deposited one. The microstructure of heat-treated LSF 17-4 PH is thin homogeneous lath martensite and there are more second phase strengthening particles percipitated in the subtrate as well. These second phase strengthening particles should be NbC and M7C3, M23C6 carbides. After heat-treatment,the strength and hardness of samples are slightly improved, while the plasticity is highly improved. In addition, the tensile strength and plastcity of the heat-treated samples are higher than forging bar standard, meanwhile, the yield strength is slightly lower than forging bar standard. The analysis on fractography shows that both the as-deposited and heat-treated LSF 17-4 PH steel exhibit toughness fracture and the second phase particles which consist of M7C3 carbides are the resource of microcavities and dimples.