The influence of impurity grain boundary segregation on mechanical properties of low-alloy Cr-Ni-Mo steels is discussed. Mechanisms of impurity and alloying effects on grain boundary embrittlement of low-alloy steels are considered. An approach for estimation of radiation enhanced grain boundary segregation kinetics is proposed. The approach takes into account the reduction of dislocation density (from 1×1014 to 1×1013 m-2) due to irradiation that was observed for low-alloy Cr-Ni-Mo steels. The influence of collision cascades on the radiation damage is also considered. Auger spectroscopy is used for grain boundary segregation detection. The contribution of grain boundary embrittlement to mechanical property degradation due to neutron irradiation is assessed.