Abstract: The influence of the temperature and strain rate on fracture toughness was studied for pressure vessel steel 20 g. The fracture toughness Ji was measured at 400°C and 500°C under four constant strains rates cases. The fracture toughness Ji decreases greatly as the strain rate increases and the fracture toughness Ji value of the same strain rate at 400°C is less than that at 500°C on loading condition. The appropriate evaluation to the pressure vessel safety at high temperature condition should consider the rule of the fracture toughness varied with temperature and strain rate, and measure the related Ji, value by high temperature fracture toughness experiment.