Abstract Hydrogen diffusion plays a key role in hydrogen embrittlement which leads to the failure of metals. In the present work, hydrogen diffusions in Fe and α-Fe2O3 were studied by first-principles calculations. The results show the energy barrier of hydrogen diffusion in pure Fe is small and hydrogen atom has a high diffusivity at ambient temperature. In contrast, the hydrogen in α-Fe2O3 has a much higher diffusion energy barrier and doesn't diffuse below 600 K. The present study indicates the in-situ oxidation of iron could be an effective hydrogen diffusion barrier to prevent the hydrogen embrittlement issue in metals.