The mitigation of residual stress in additive manufactured metal parts through post-heat treatment helps to enhance life during service. The present work aims to investigate the effect of the high-temperature heat treatment at 1065°C on microstructural changes, residual stress patterns, and hardness of Inconel 718 metal blocks fabricated through the Laser powder bed fusion (L-PBF) process. The non-destructive X-ray diffraction technique known as cos α method was used to measure the surface residual stress by capturing the Debye ring. It was found that the existence of columnar dendrites with laves phases in the interdendritic region along the building direction was dissolved after heat treatment. The resultant microstructure showed homogeneous equiaxed grains with the formation of annealing twins along with strengthening phases (γ’ and γ’’) and tiny metal carbides on the grain boundaries. Further, the residual stress magnitude in the as-built sample was found to be 77% higher on the corners compared to the center region of the top surface. The average value of residual stress on the top surface was found to be 35.5% lesser than the lateral side surface of the sample. It was observed that the distribution of residual stress is not uniform in as-built condition. The heat-treated sample showed uniform distribution of compressive residual stresses in all the locations. This phenomenon happened due to the increase in diffusion of atoms present in the high-stress regions migrated to low-stress regions till attain their equilibrium condition. During this action, the stresses present in the grain interiors were altered considerably and resulted in complete recrystallization with the precipitation of more strengthening phases and annealing twins. The high-temperature post-heat treatment also affects the hardness leads to 23.03% increase in average microhardness mainly induced resistance to dislocation motion by precipitation of strengthening phases in the γ matrix.