Experiments were conducted to evaluate the influence of equal-channel angular pressing (ECAP) on the microstructure and mechanical properties of 17-4PH steel. The 17-4PH steel was solution-treated at 1040°C for 1h followed by oil cooling before ECAP process. Both the as solution-treated and ECAPed specimens were tempered at 460°C and 610°C for 2h, respectively. Tensile, impact and hardness properties as well as the microstructure and fracture mode were systematically studied. The results show that both the tensile strength and hardness are increased by ECAP process, the elongation is deceased but the Charpy V-notch energy absorption is increased by ECAP process when the tempering treatment is at a low temperature of 460°C, while the result is just reverse when the tempering temperature is 610°C. SEM observation reveals that the ECAP process promotes the precipitation of M23C6-type carbide obviously. The relations among ECAP process, microstructure, mechanical properties and fracture modes are discussed.