At high plastic strain level, the work-hardening and the fracture behavior of metals are accompanied by a significant change in the dislocation structure. In order to understand the hydrogen effect on the dislocation structure evolution processes and its relation to the hydrogen-induced fracture mode transition, the microstructures in pure nickel processed by high-pressure torsion in the presence of hydrogen and a ferritic-pearlitic low carbon steel fatigued in a hydrogen gas environment were investigated. At sites of interest, electron transparency foils were extracted using focused-ion beam, and the microstructures were examined by a zone-axis bright-field imaging method in a scanning transmission electron microscope. The local crystalline orientations of the dislocation structure were obtained by using electron diffraction analysis. In both materials, the sizes of the dislocation structure unit were refined in the presence of hydrogen. However, on normalizing the cell size distributions by the mean cell sizes, the data in the absence and presence of hydrogen obeyed the same distribution function, which indicated the same evolution pathway for these structures formed in different environments. In comparison to the structure formed in air, hydrogen modified the misorientation angle gradient, and increased the misorientation angle between structure units at a high level of plastic strain. A general evolutionary law for work-hardening at a high strain level in the presence of hydrogen was proposed based on the hydrogen shielding effect on the dislocation stress field. By combining these results with previous studies on dislocation structure in the vicinity of fracture surface, this work suggests that hydrogen-enhanced localized plasticity accelerates the evolution process of dislocation structure and plays an important role in the fracture process.