The steel A533-B is a low alloy steel used as a structural material for the construction of light water nuclear reactor pressure vessels. It is normally cladded with the 308 austenitic stainless steel weld metal overlay for protection against extension of surface flaws generating during operation. In this investigation the impact toughness of the A533-B steel and its 308 weld metal were evaluated. The microstructure of the A533-B steel consisted of bainitic structure while that of the 308 weld metal comprised austenite with ferrite phase of about 5%. The impact toughness was assessed using instrumented impact testing machine at temperatures between -196 and 300 C. The determined impact toughness parameters involved the total impact fracture energy, the energy expended in the fracture crack initiation and crack propagation, the ductile to brittle transition temperature as well as the dynamic yield strength. The A533-B steel exhibited distinct ductile to brittle transition behavior with superior resistance to ductile fracture at high test temperatures. In contrast, the 308 weld metal did not display such transition. However, it showed low energy ductile fracture performance with evident resistance to brittle fracture even at low test temperatures. The findings were substantiated with the load-time traces derived from the instrumented impact tests as well as the fracture surface morphology. The results were discussed in relation to the difference in microstructure and flow properties of the examined steels.