It is well known that hydrogen weakens strengths of metals, and this phenomenon is called hydrogen embrittlement Despite the extensive investigation concerning hydrogen related fractures, the mechanism has not been enough clarified yet. In this study, we applied the molecular dynamics method to the mode I crack growth in alpha-Fe single crystals with and without hydrogen, and analyzed the hydrogen effects from atomistic viewpoints. We estimated the hydrogen trap energy in the vicinity of an edge dislocation in order to clarify the distribution of hydrogen atoms, using the molecular statics method We also evaluated the energy barrier for dislocation motion under a low hydrogen concentration. Based on these results, we propose a mechanism for hydrogen embrittlement of alpha-Fe under monotonic loading. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd All rights reserved.