Additive manufacturing is a technology that creates parts, components or repairs them through layer-by-layer deposition of material using a sliced CAD model of the desired geometry. Maraging steels are types of ultra-strength steel that are used in tool industry and aerospace applications. The aim of this paper is to characterize samples made up of M300 maraging steel, which are manufactured using Direct Energy Deposition (DED), one of the additive manufacturing techniques. Initially, M300 samples were subjected to different heat treatments at 480oC and 550oC for 2,4 and 6 hours. Porosity analysis and microstructure characterization of all heat treated samples were done and compared to the as-built sample.