Charpy impact specimens are preferred to characterize the deterioration of steel in irradiation surveillance program in nuclear industry. But the fracture toughness cannot be acquired by the Charpy impact experiment directly. In this paper, the reference temperature T0MC of Q345R and A508-3 are determined by experiment and empirical formulas between Charpy impact energy (CVN) and fracture toughness. The measured values of T0MC are compared with predicted values. Empirical formulas combined with minimum design metal temperature (MDMT) curve is put forward to determine the minimum design metal temperature of ferritic steel conservatively.