Hydrogen embrittlement has been extensively studied for decades; however, the atomistic origins of hydrogen embrittlement remain debated. Here, we studied the role of body-centered cubic (bcc) to face-centered cubic (fcc) phase transition at the crack tip on hydrogen embrittlement in alpha iron at room temperature. Molecular dynamics simulations were performed to examine the propagation of [10 (1) over bar](101) crack. The results indicate that in the absence of hydrogen atoms, the phase transition could release the strain energy and postpone the crack propagation. Hydrogen atoms would form a nanosize hydrogen-rich region ahead of the crack tip and inhibit phase transition, thus enhancing brittle crack growth. A general expression is provided to predict the hydrogen concentration beyond which the phase transition does not occur. In addition, a simplified crack growth model is proposed to predict the risk level during the operation. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.