Abstract: Nonequilibrium statistical method is used to study the helium embrittlement of metals under neutron irradiation and high temperature. The evolution equation of helium bubbles is established. On the basis of discussion about the micromechanisms of nucleation and growth of helium bubbles the nucleation rate and the growth rate of helium bubbles, the fluctuation growth coefficient are obtained, and consequently, the density distribution function of helium bubbles and the critical radius of helium bubbles for helium embrittlement fracture are derived theoretically.