Fracture toughness is an important property to characterize the resistance of metals against crack initiation and growth. It has been found that hydrogen gas has a detrimental effect on fracture toughness of many metals. It is a very difficult and dangerous task to measure experimentally the fracture toughness of metals in hydrogen gas, particularly in high-pressure hydrogen gas. In this paper, a model was developed to quantify the effect of hydrogen on the fracture toughness of metals based on the theories of stressinduced hydrogen diffusion, hydrogen enhanced decohesion and fracture mechanics. Experimental data in some literatures were used to verify the model, and a good agreement was obtained. The model can be used to predict fracture toughness of metals in hydrogen gas, or to extrapolate the fracture toughness of steels in the high pressure hydrogen gas based on the fracture toughness obtained experimentally in the low pressure hydrogen gas.