Abstract   
                The present work has an attempt to possibly obtain the most suitable and practicable repair-condition, which could provide the optimal microstructural characteristics by rejuvenation method of hot isostatic pressing (HIP) followed by standard heat treatment for long-term serviced gas turbine blades, a casting nickel base superalloy grade IN-738 operated by Electricity Generating Authority of Thailand (EGAT). The hot isostatic pressing could mostly heal any internal structural voids and cracks (by means of sintering), which were generated during service by creep mechanism. It was found that no any microcrack was observed after longer times of HIP process such as for 3 - 4 hrs. HIP time period. Furthermore, the higher of HIP temperature and/or the longer of HIP time provided the higher of efficiency for microvoid or porosity closing. However, microvoids were still found even after 4 hrs.-HIP process at 1200?C but in very small amount. Therefore, both size and amount of remain microvoids should be considered comparing to both obtained original material and after long-term service one. Furthermore, during solution treatment, coarse carbides and over-exposed gamma prime precipitates, which formed previously at the grain boundaries during service, would dissolve into the matrix. Then specimens will be processed through a series of precipitation aging, which re-precipitates the strengthening phase to form the proper morphology in size and shape as well as distribution that is almost similar to the new one. Metallurgical examination of the microstructure had been performed by utilizing optical microscope and scanning electron microscope after hot isostatic pressing and heat treatment to evaluate the micro-defects elimination.