Abstract: A well formed 18Ni maraging steel thin-walled part was prepared by the gas metal arc additive manufactring. The microstructure and mechanical properties of the as-deposited and heat treated component were studied. The results showed that the microstructure of the component was mainly cellular dendrite, and the microstructure and mechanical properties of the as-built component in different positions were various: the top of the thin-wall was martensite, and the average hardness was 360 HV. The hardness of the middle part was 468 HV, slightly higher than that of the bottom part (437 HV). The tensile strength of the component (1375 MPa) in the vertical direction was about 28.3% higher than it in the horizontal direction (1072 MPa), and the corresponding elongation was 1.1% and 0.8% respectively. After the solution heat treatment at 825 ℃ for 1 h, the precipitates of the part were remelted into austenite and the hardness decreased (the average value was 328 HV) with little variation. The tensile strength in the vertical direction (1025 MPa) were equivalent the horizontal direction (1034 MPa) and the elongation was 6% and 14%, respectively.