The influence of aging temperature on phase transformation and mechanical properties of weld metal of maraging steel(grade C300)was studied.Microstructure was analyzed by means of optical microscopy,transmission electron microscopy,scanning electron microscopy and energy dispersive spectrum analysis.Gibbs free energy of Ni3 Ti and Fe2 Mo at different temperature was calculated by Thermal-calc software.The microstructure of weld metal in aswelded state is martensite.The yield strength of weld metal after 430 ℃ aging process may increase to 1 561 MPa from 890 MPa in as-welded state,which is ascribed to the formation of spinodal constitute and GP zones.After 480 ℃aging process,there are great deal of Ni3 Ti precipitates in the martensite matrix and 10%reverted austenite phase in the cellular grain boundary,and the yield strength increases to 1 801 MPa.After aging process at 580 ℃,there are many Fe2 Mo precipitates in the martensite matrix and 30% reverted austenite phase in the cellular grain boundary,and the yield strength is 1 329 MPa,which is the lowest among the three cases.The phase transformation may also influence the toughness.It is found that precipitates make the toughness decrease and reverted austenite increases it.The mechanism of phase transformation on strength and toughness is discussed. The influence of aging temperature on phase transformation and mechanical properties of weld metal of maraging steel (grade C300) was studied. Microstructure was analyzed by means of optical microscopy, transmission electron microscopy, scanning electron microscopy and energy dispersive spectrum analysis. Gibbs free energy of Ni3 Ti and Fe2 Mo at different temperature was calculated by Thermal-calc software. The microstructure of weld metal in as welded state is martensite. The yield strength of weld metal after 430 ℃ aging process may increase to 1 561 MPa from 890 MPa in as- welded state, which is ascribed to the formation of spinodal constitutes and GP zones. After 480 ° C aging process, there are great deal of Ni3 Ti precipitates in the martensite matrix and 10% reverted austenite phase in the cellular grain boundary, and the yield strength increases to 1 801 MPa. After aging process at 580 ° C, there are many Fe 2 Mo precipitates in the martensite matrix and 30% reverted austenite phase in the cell ular grain boundary, and the yield strength is 1 329 MPa, which is the lowest among the three cases. the phase transformation may also influence the toughness. It is found that precipitates make the toughness decrease and reverted austenite increases it. mechanism. transformation on strength and toughness is discussed.