A simple, physically based model is developed to quantitatively predict the degradation of hydrogen on the crack growth critical stress intensity factor (CSIF) of metals. The model is formulated by combining a microscopically shielded Griffith criterion (MSGC) model for plasticity-induced cleavage fracture and thermodynamics decohesion (TDD) theory for hydrogen-enhanced interface decohesion. The hydrogen-influenced CSIF is described as a function of the intrinsic CSIF (hydrogen-free), initial hydrogen concentration (solubility), hydrogen trap binding energy and crack tip stress. All parameters in the model can be determined with a physical basis and the model is successfully validated by comparison with published experimental data.