The pressure vessel steel A533-B used in the nuclear power reactors was heat treated to simulate thermal circumstances similar to those that could develop during a loss of coolant incident. The applied heat treatment resulted in phase transformation of the steel from tempered bainite structure into untempered lath martensite structure. The impact properties of the steel in the as-received condition and after heat treatment were evaluated. The results showed that the as- received steel has experienced serious degradation in the impact properties after exposure to heat treatment. The ductile to brittle transition temperature increased by 100oC and the upper-shelf energy was lowered by 30%. Room temperature impact test results showed 100% ductile fracture behavior of the steel with a total fracture energy of 200 J. After heat treatment, the steel displayed 100 % brittle fracture behavior with a total fracture energy of 12 J. Load-time curves of the instrumented impact tests showed that the decrease in the impact fracture energy was mainly due to the decrease in the fracture propagation energy than in the fracture initiation energy. Fracture surface morphology of specimens tested at room temperature revealed ductile microvoid coalescence fracture features for the as-received steel and brittle cleavage fracture features for the steel after exposure to heat treatment. The resulted embrittlement of the steel was discussed in relation to the phase transformation of the steel microstructure in consequence of the applied heat treatment.