It has been reported that the high temperature austenitisation heat treatments have resulted in improving the fracture toughness of several structural steels. This is contrary to the conventional understanding of the relationship between larger prior austenite grain sizes, associated with high austenitising temperature, and fracture toughness. This anomalous increase in fracture toughness has some important practical significance to welded joints where the material adjacent to fusion zone experiences high austenitising temperatures resulting in a coarse grain zone. This investigation has been undertaken to verify the effects of microstructure and grain size on fracture toughness in a typical pressure vessel steel that is extensively welded during fabrications. A range of simulated weld heat affected zone microstructures were produced by austenitising the specimens at various temperatures followed by quenching at three different cooling rates. The trends of variations in properties, such as, fracture toughness are explained in terms of grain size, and microstructural analyses. It was observed that the specimens provided with low temperature austenitisation followed by air-quench possess a better fracture toughness than the base metal.