A theory of creep fracture has been constructed by means of the concep
t and the method of nonequilibrium statistical physics. In this paper,
the cavity evolution equation is given;the cavity distribution functi
on, the cavity distribution probability, the creep fracture life of ma
terials and the famous Monkman-Grant empirical relation are derived on
the basis of the two mechanisms of the constrained cavity growth and
the unconstrained cavity growth, respectively.