The effects of hydrogen addition on superplastic forming behavior of Ti-55 alloy were investigated through gas-bulging experiment. It was found that reasonable hydrogen content can significantly improve the superplastic forming properties. The maximum ratio of height to radius reaches 1.58 for hydrogenated specimen (with 0.3 wt%H) at 825 degrees C, a drop by about 100 degrees C when compared to as-received alloy. This is due to the increased volume fraction of extremely deformable beta phase, with alpha/beta phase ratio close to 1:1, as well as microstructural refinement and dynamic recrystallization in hydrogenated alloy. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.