An investigation of a rolled Ti6Al4V alloy after thermo hydrogen treatment was performed. The effect of hydrogen content on the types and amount of lattice defects, the microstructure refinement after hydrogenation dehydrogenation processing and the refining mechanisms were studied. The results show that the types of defects are at first vacancies and dislocations, and then they are mainly dislocations with increasing of hydrogen content. The amount of defects increases gradually with increasing of hydrogen content. After thermo hydrogen treatment, the rolled microstructure of Ti6Al4V alloy is refined. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.