Abstract: Laser powder bed fusion (LPBF) exhibits significant advantages in the manufacturing of high-entropy alloys (HEAs) with complex shapes and good strength-toughness properties. To further enhance the mechanical strength of HEAs, adding Nb element is an effective way which could enhance the solid solution strengthening and second phase strengthening effects. However, the structure features of second phase formed during LPBF is prone to brittle fracture, resulting in the significant decrease in the toughness properties of HEAs. Therefore, it is of great significance to study the effect of heat treatment post-processing on the microstructure and properties of HEAs fabricated by LPBF. The CoCrFeMnNiNb_0.15 HEAs were successfully prepared by LPBF first, and then the evolution of phase structure, mechanical properties and fracture morphology were studied after heat treatment. The results show that the heat treatment prompts the structure transformation of Nb-rich Laves phase from the continuous network to the fine and dispersively-distributed particles, and partial Nb element precipitates from the matrix structure of the FCC phase. Besides, the matrix grains also grow up along the (111) crystal orientation. With the increase of heat treatment temperature, the Laves phase and the matrix structure become coarsened gradually. Due to the weakened solid solution strengthening, grain boundary strengthening and second phase strengthening effects after heat treatment, the plastic deformation capability of the FCC phase is improved, thus resulting in the failure mode changed from brittle fracture to ductile fracture. When the heat treatment temperature is 980 ℃, the CoCrFeMnNiNb_0.15 HEAs can obtain the well-matched strength-toughness properties, which shows the product of strength and elongation of 13.2 GPa%, ultimate tensile strength of (892 ± 13.4) MPa, and elongation rate of 17.4%.