The effect of heat treatment on the solidification microstructure of the laser solid formed DZ125 superalloy is investigated. The results show that, with the increase of solution treatment temperature, the more primary γ′ particles are dissolved and these particles can be completely dissolved after treatment at 1240 ℃ for 2 h. There occurs solid-state phase transition for the Ni5Hf phase and MC(1) carbides at high-temperature insulation. When treated at 1180 ℃ for 2 h, the Ni5Hf phase can be completely decomposed and the released Hf subsequently reacts with C atoms in solid solution matrix to form MC(2) carbides. Some MC(1) carbides transform into M23C6 or M6C carbides when maintained at 1000 ℃ for 12 h. The coarsening behavior and the empirical distribution function of the γ′ phase are consistent with those given by the Lifshitz-Slyozov-Wagner (LSW) theory when aged at 1100 ℃ and 870 ℃ after the complete solution heat treatment. The fitted coarsening activation energy of the γ′ phase is 231.43 kJ·mol-1 and the coarsening of γ′ particles is controlled by the diffusion of Al and Ti species in Ni.