The hydrogen absorption characteristics and microstructural evolution of TC21 titanium alloy were investigated by kinetic model analysis, optical microscopy （OM） and X-ray diffraction （XRD）. The results show that the hydrogen absorption reaction occurred during the hydrogen absorption process of TC21 titanium alloy can be divided into two different stages according to the hydrogen absorption kinetics. After hydrogenation, the microstructure of TC21 titanium alloy changes obviously. Just a little hydrogen will change the contrast of transformedβphase. The contrast ofα phase darkens when the hydrogen content in TC21 titanium alloy exceeds 0.5% （mass fraction）. The phase/grain boundaries become ambiguous or even vanished, andβ phase becomes the main phase instead ofα phase when the hydrogen content reaches 0.625%. Moreover,α phase disappears when the hydrogen content reaches 1.065%. Additionally, the XRD analysis shows that α＇ martensite and FCCδ hydride appear in the hydrogenated alloy. According to the microstructures and XRD analysis, the schematic diagrams of hydrogen diffusion process in TC21 titanium alloy were established.
