The effects of hydrogen addition (0.12 wt.% H) on microstructural evolution in Ti-0.3Mo-0.8Ni alloy argon-arc welded joints have been investigated using optical microscope (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) to reveal the influence of hydrogen on the characteristics of defect-free titanium alloy welded joints. The results show that hydride precipitation changed the initial microstructure of the welded joints, the increase of hydrogen content was more favorable for the beta phase precipitation. Face centered cubic δ hydride evenly distributed in the hydrogenated 0.12 wt.% H welded joints, the lamellar δ hydride could only precipitate from the lamellae alpha, and not from the transformed beta phase. Formation of δ hydride was associated with the result of αH phase separation reaction: αH→α (H lean region) + δ (H rich region), and H rich regions finally transformed to the δ phase. The dislocation distribution was heterogeneous and there was a relatively high density of dislocations in the vicinity of the precipitated δ hydride, which were caused by the lattice distortion due to the stress field generated by the hydride precipitation.