Abstract: 17-4PH stainless steel was fabricated by selective laser melting (SLM). The microstructure of the as-built and solution heat treated 17-4PH was analyzed by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The relationship between microstructure and dynamic fracture behavior was investigated by performing instrumented impact test. Absorbed impact energies related to crack initiation, stable and unstable propagation were calculated and the dynamic J−R curves were estimated.The results demonstrate that as-built 17-4PH stainless steel mainly consists of coarse columnar δ ferrite grains growing along the building direction with <100> texture and fine martensitic grains with random orientation. A small amount of austinite can also be found in the as-built sample. As-built 17-4PH stainless steel displays low resistance to crack initiation and propagation, resulting in marginally rising J−R curve and quasi-cleavage fracture. After solution heat treatment, the retained ferrite transforms into martensite and microstructural anisotropy can be eliminated. The impact toughness is 1 times higher than that in as-built conditions and the dynamic J−R curve rises steeply, indicating superior dynamic mechanical properties. Fracture surfaces revealed that the inferior dynamic fracture toughness of as-built 17-4PH stainless steel can be attributed to the weak boundaries between the coarse δ ferrite grains and surrounding fine martensite grains.