Based on ideas about physical nature of brittle and ductile fractures of RPV steels, the criterion for determining the critical temperature of brittleness from the Charpy impact test data is proposed. As shown in the first approximation, the criterion of constancy of local plastic deformation at head of stress raiser can be used to specify the level of impact toughness $KCV_{th}$ depending on the yield stress $\sigma_{0.2}$. As theoretically proved, the threshold level of impact toughness is not an unambiguous function of $\sigma_{0.2}$. An additional influence is exerted by factors, which characterize the ability of RPV metal to resist brittle fracture under stress concentration conditions, in particular, the brittle strength $R_{MC}$. An approximate analytical relationship is derived, which allows predicting the level of $KCV_{th}$ considering the strength of the RPV metal. In the range of yield strength 400–690 MPa, this relationship is consistent with the $KCV_{th}$ values, which are given in the PNAE G-7-002-86 standard.