The principle of minimun energy dissipation and its general procedures to establish strength criterion of materials
have been intoduced.With the widely accepted viewpoints that strength criteria express the energy accumulation of material and utilizing the viscoelastic fracture theory the strength criteria for crack creep are deduced.For the sake of contrast these viscoelastic equations are also obtained directly from strength criterion of elastic fracture theory in form of strain energy density.Therefore creep stability of fracture in several kinds of viscoelastic material is analyzed