One of the issues related to long-term operation of LWRs is the potential for the thermal embrittlement of A533B steel. In this study, A533B steel, also known as reference surveillance HSST Plate 01, has been examined in as-received condition and after high austenization heat treatment. The goal of the heat treatment was to obtain extremely large grains size that were known for high susceptibility to temper embrittlement. After that, both materials were thermally aged at 450C for 2000 hours. Tensile, Charpy V-notch impact, and static fracture toughness tests with very wide suite of microstructural characterization techniques (Auger electron spectroscopy, TEM, SEM fractrography and microhardness measurements) have been applied in this study. This paper provides detailed description of effect of grain size on thermal aging response of A533B steel by means of various mechanical and microstructural techniques.