In order to investigate the mechanism of brittle-striation formation in low-carbon steel fatigued in hydrogen gas, fractographic observations were performed after programmed load fatigue tests and crystallographic cross-sectional observations were made using high-voltage electron microscopy. The results indicate that the processes of brittle-striation processes were microscopically similar to the normal ductile tearing process. In particular, they consist of a crack-tip blunting process and a stable ductile crack growth process. Hydrogen enhanced the occurrence of microscopic ductile tearing processes. Furthermore, the localization of hydrogen near the crack tip enhanced this process further. Based on the experimental results, a mechanical model for the brittle-striation-formation was proposed in which the microscopic ductile tearing fracture was enhanced by local hydrogen.