Abstract: FeCoNiAl medium-entropy alloys (MEAs) have broad application prospects due to their unique microstructure and excellent mechanical properties.A novel FeCoNiAl MEA was manufactured by selective laser melting (SLM) technique.The effects of aging temperature on the microstructure and hardness of the alloy was studied. The strengthening mechanism of the FeCoNiAl MEA prepared by SLM was revealed. The results show that the as-built alloy is a single BCC phase structure with a hardness of 31.2 HRC.After aging treatment, a large number of nano-scale Ni₃Al and Ni₃Fe are precipitated in grain boundaries and grains. With the increase of the heat treatment temperature, the content of Ni₃Fe phase in the alloy increases gradually, and the hardness of the alloy shows a trend of increasing first and then decreasing, and it reaches the highest value of 44.3 HRC after aging treatment at 430 ℃for 2 h. Increasing the aging temperature to 600 ℃, the Ni₃Fe precipitates change from point-like and needle-like to coarse dendritic, the FCC phase content reaches 33.9%, and the aging hardness of the alloy decreases to 31.9 HRC. The precipitation strengthening caused by the precipitation of nano-scale Ni₃Fe and Ni₃Al during heat treatment is the main reason for the increase of hardness of the FeCoNiAl MEA.