Stainless steel (SS) alloys produced by Laser-based Powder Bed Fusion (LPBF) process offers comparable and sometime improved mechanical properties respect to conventionally processed materials. Some of these steels have been extensively studied in the last decade, however additively manufactured martensitic SS, as AISI 420, need further research in characterizing their post built quality and mechanical behaviour. This lack of information on martensitic SS is not consistent with their growing demand in the automotive, medical and aerospace industries due to their good corrosion resistance, high hardness and good tensile properties. Selection of the appropriate process parameters and post treatments plays a fundamental role in determining final properties. For this reason, the effect of LPBF process parameters and different heat treatments on density, defects characteristics and location, roughness and mechanical properties of AISI 420 were investigated in this paper. A first experimental campaign was carried out to establish different set of suitable process parameters for industrial applications. Starting from this result, the detected defects properties was investigated by Computed Tomography (CT) scans. Dimensions, sphericity and distributions of defects inside the volume were analysed and compared between samples manufactured with different parameters. In the second part of the paper, the influence of process and post process conditions on mechanical properties was investigated. The final presented results establish a correlation between the involved production cycle and the resulting properties of LPBF AISI 420 specimens.