Ti-6Al-4V (Ti64) sheet specimens were cathodically hydrogenated in sulfuric acid solution at ambient conditions. The hydrogenated specimens were then sent to go through the designed thermohydrogen processing (THP) twice to obtain a nano-sized grain structure. The average grain size of resulted microstructure was found to be 10-20 nm obtained by TEM. Qualitative and quantitative analyses performed by employing X-ray diffractometry (XRD) and elemental analysis (EA) showed that the addition of AS(2)O(3) as hydrogenation promoter in electrolyte significantly increased the hydrogen uptake. The high concentration of hydrogen arising from promoter action is the key factor in grain refinement. The optimal processing parameter found for grain-refining Ti64 was: (1) electrolytic hydrogenation at 100 mA cm(-2) for 3 h in 1 N H2SO4(aq) by adding 0.1 g L-1 AS(2)O(3); (2) P transformation carried out at 850 degrees C for 1 h in air furnace, followed by a furnace cooling to 590 degrees C and held for 6 h; (3) oxide film removed and then dehydrogenated at 650 degrees C and 1.0 x 10(-6) Torr for 10 h; (4) repeated the same processes once more. (C) 2008 Elsevier B.V. All rights reserved.