Abstract: The austenitic stainless steel 316L was fabricated by high power MIG arc additive manufacturing and its microstructure evolution, mechanical properties and fracture behavior were investigated. Results show that there are columnar grains forming in the MIG arc additive manufacturing 316L. The microstructure consists of vermicular δ and σ phases within γ matrix. The δ distributed both at grain boundaries and in the grains can strengthen the steel. With the arc power increasing from 3 763 W to 8 400 W, δ phases decrease and σ phases increase, as well as grain size increases. That leads to the ultimate tensile strength decrease from 578 MPa to 533 MPa, yield strength decrease from 310 MPa to 235 MPa, elongation decrease from 53% to 44%, reduction of area decrease from 67% to 60%. A pronounced feature is that the fracture type transfers from trans-granular dimple fracture to inter-granular dimpled fracture at the arc power of 8 400 W.