The alpha + beta titanium alloy, Ti-6Al-4V, was thermohydrogen processed with 0.3 wt% hydrogen. The friction stir welding characteristics of Ti-6Al-4V alloy and the hydrogenated alloy with 0.3 wt% hydrogen were investigated in contrast. The results showed that welding parameters range for the unhydrogenated alloy was narrow and flash was prone to occur in the welding process. Hydrogenation could help to widen welding parameters range, improve weld appearance, and increase service life of pin tool. The reason was attributed to the hydrogen-enhanced thermoplasticity of titanium alloys, while the fundamental cause was the hydrogen-induced microstructural evolution in titanium alloys. Hydrogen almost did not escape from the hydrogenated alloy during the friction stir welding and could be successfully removed through post-weld vacuum annealing. The mechanism of hydrogen-induced effect during the friction stir welding and post-weld dehydrogenation was discussed in detail. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.