When studying the growth of triaxial-stress-state crack, the plastic work rate consumed in the process zone at crack tip should be described clearly. Fracture for a cracked specimen is related to rupture of a simple uniaxial tensile specimen by nature. By equaling the crack tip stress-strain field to uniaxial stress-strain field, it is obtained that the plastic work rate consumed in the process zone at crack tip can be expressed by the plastic energy density WF multiplied by the equivalent size of the process zone hF. The plastic energy density WF can be expressed by virtue of uniaxial true stress-strain curve. And analytical form of hF for triaxial stress state is presented by recourse to three-dimensional analyses on material behaviors. The expression of plastic work rate consumed in the process zone is verified by the predicting fracture toughness.