Grain boundary character distribution-optimized (GBCD) Type 316 corresponding austenitic stainless steel and its cold-worked form (GBCD+CW) are prospective materials to be considered for next generation nuclear energy systems. Specimens of these steels were thermally-aged at 973 K for 1 and 100 h and then examined by transmission electron microscopy (TEM) to evaluate microstructural stability during heat treatment high temperature. TEM results revealed that microstructures of both specimens types prior to ageing contained step-wise boundaries which were composed of coincidence site lattice (CSL) boundaries. The GBCD+CW specimens had dislocation cells and networks as well as deformation twins whereas the GBCD one possessed few dislocations. After thermal ageing, the precipitates were formed on not only random grain boundaries but also on dislocations, and they contribute to prevent significant microstructural change such as recrystallization and dislocation recovery.