The general peculiarities of grain boundary fracture of irradiated materials are discussed. Such physical phenomena as high temperature helium embrittlement of structural fusion materials and irradiation-assisted stress corrosion cracking of austenitic alloys in nuclear reactor core components under neutron irradiation are characterized by the ductility loss and intergranular fracture. In fission and fusion structural materials due to (n, α) and (n, p) reactions, high concentrations of helium and hydrogen can be generated. The effects of accumulation of gaseous (helium, hydrogen) atoms, segregation of solute impurity atoms at grain boundaries (GBs) and grain boundary sliding process of fracture of irradiated materials are discussed. At high temperatures, in irradiated materials containing helium atoms the system of helium bubbles is formed. In stressed materials, the kinetics of helium bubble growth on GBs is considered. The effect of grain boundary bubbles and bubbles on triple grain junctions (TGJs) on the fracture of irradiated alloys is analyzed elsewhere. The fracture process in these conditions is modeled in terms of interaction of bubble arrays on TGJs with a pile up of dislocations during creep and grain boundary sliding. Fracture criteria for a material are formulated in their dependence upon bubble density and size, gas content in bubbles on TGJs and time scale of crack growth.