The relationships among the hydrogen-induced phase transformation, grain refinement and improvement of mechanical properties of Ti-6Al-4V alloys are investigated. For this purpose, the decomposition of the alpha '' martensite and metastable beta phase of hydrogenated Ti-6Al-4V alloys was investigated first. As a result, it is found that the key role leading to grain refinement is the decomposition of metastable phase. on the other hand, the precipitation of the TiH(2) hydride under lower temperature aging shows no significant refining effect after dehydrogenation. Mechanical properties of the materials after dehydrogenation show that the decomposition of the metastable phase strengthens the materials. In particular, specimens containing 0.45 wt% H in their fully martensites structures show a marked increase in strength. However, a fine grain and comprehensive mechanical properties are obtained only in specimens containing 0.8 wt% H. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.