300M ultra-high strength steel has been widely used in critical structural components for aviation and aerospace vehicles,owing to its high strength,excellent transverse plasticity,fracture toughness and fatigue resistance.Herein,low and high power selective laser melting (SLM) of 300M steel and their microstructural evolution and mechanical properties have been reported.The results show that the opti-mal energy density range with the highest relative density for SLMed 300M steel is between 60 and 160J/mm3.Furthermore,molten pools for deposition exhibit a conduction mode with semi-elliptical shape at a lower laser power of 300 ～ 600 W but a keyhole mode with “U” shape at a higher laser power of 800～1900 W.The heterogeneous microstructure of as-built samples is characterized by a skin-core struc-ture which is that tempered troostite with the coarse non-equiaxed grains in the molten pool is wrapped by tempered sorbite with the fine equiaxed grains in the heat-affected zone.The skin-core structure of SLMed 300M steel has the characteristics of hard inside and soft outside.The average microhardness of samples varies from 385 to 341 HV when laser power increases from 300 to 1900W.Interestingly,ultimate tensile strength (1156-1193 MPa) and yield tensile strength (1085-1145 MPa) of dense samples fabricated at different laser powers vary marginally.But,the elongation (6.8-9.1％) of SLMed 300M steel is greatly affected by the laser power.