Gas turbine blades for power generation are generally made of Ni-base superalloys that feature excellent resistance to high temperature and corrosion. Since the blade material properties degrade due to long-term service operation of a gas turbine, the blades need to be replaced periodically. Using service degraded blades, we conducted research on the recovery of the blade material properties. To check whether the properties of the degraded blade materials had recovered or not, we applied hot isostatic pressing (HIP)-combined heat treatment technique in our research, and then inspected material properties before and after the heat treatment by various non-destructive and destructive tests. As a result, the heat treatment could recover metallurgical and mechanical properties of the degraded blades. After the treatment was applied to the degraded blades, those blades were reinstalled in a gas turbine, and the operation has restarted. At the time of regular periodical inspection, various non-destructive and destructive tests to verify their degradation showed no cracks or deformation in any of those blades, with very little change in microstructure and mechanical properties. No acceleration in degradation rate was revealed, and we further confirmed that those treated blades installed in the gas turbine were completely operational. Through our research, trial operation and subsequent damage inspection, it was confirmed that HIP-combined heat treatment could be a useful service-life extension method for gas turbine blades.