Hydrogen-inducing nanovoids in hydrogenated 310 stainless steel was investigated by in situ tension in TEM. It was found experimentally that hydrogen-induced cracking happened through nanovoid nucleation and then quasi-cleavage along {1 1 1} planes when C-H is high. Otherwise, in the case of low C-H, hydrogen-enhanced ductile fracture through hydrogen-induced microvoid nucleation, growth and connection. A new model was proposed based on the present experiments. Dislocations breakaway from defect atmospheres and move away from the DFZ, leaving vacancy and hydrogen clusters along {1 1 1} planes. Hydrogen tends to combine with vacancy clusters and initiate nanovoids along {1 1 1} planes. Dense nanovoids connect each other, resulting in brittle cracking. Scattered nanovoids grow into microvoids or even macrovoids, leading to ductile fracture.