Abstract: A novel FeCoNi medium entropy alloy was fabricated by selective laser melting (SLM) additive manufacturing technology. Effect of heat treatment on microstructure and hardness of the alloy was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), and strengthening mechanism of SLM medium entropy alloy was revealed. The results showed that relative density of deposited alloy was above 99.9%, and average grain size was 2.2 μm. Microstructure was single-phase BCC structure, and hardness was 32.6 HRC. After heat treatment at 470 ℃, dual-size Ni₃Fe intermetallic compounds precipitated at grain boundary and in grains, and hardness increased significantly to 47.6 HRC. With the further increase of heat treatment temperature, coarse FCC softening phase was formed in the alloy, and content of FCC phase gradually increased with the increase of heat treatment temperature. Content of FCC phase reached 36% after 700 ℃ heat treatment, and hardness of the alloy decreased to 36.3 HRC. Therefore, precipitation of rich-Ni intermetallic compound in BCC matrix during heat treatment was one of the main reasons for improving properties of FeCoNi medium entropy alloy.