Effect of the heat treatment, including solution treatment (ST) and aging treatment (AT), on the prior austenite grain (PAG) size, microstructure and mechanical properties of a precipitation hardening maraging stainless steel was investigated. The results indicate that the relations between PAG size and yield strength (σy) under both ST and AT conditions obey the Hall-Petch relationship. Furthermore, after ST at 1050℃for 1 h+cryogenic treated (CT) at -70℃ for 8 h+AT at 535℃ for 4 h, the tested steel showed its ultimate tensile strength (σb) and σy over 1900 MPa and 1750 MPa, respectively.