The microstructures of Ti6Al4V alloy after hydrogenation were investigated by optical microscopy(OM), X-ray diffraction(XRD) and transmission electron microscopy(TEM). The influence of hydrogenation on the hardness of α and β phases was analysed by microhardness testing. The influence of hydrogenation on alloying elements diffusion was studied by electron probe microanalysis(EPMA). The microstructural observation reveals that hydride δ (FCC structure) as well as large number of dislocations precipitate in the specimens with 0.278% and 0.514% hydrogen, and a lot of twins are found in the specimen with 0.514% hydrogen, simultaneously. The result of microhardness testing shows that the hardness of α and β phases increases synchronously with the increase of hydrogen and the hardness increment of β is larger than that of α. According to analysis of EPMA, the diffusion ability of alloy elements Al and V increases after hydrogenation. It is considered that hydrogen solution strengthening and V element diffusion are the main factors causing the hardness of α phase increase with the increase of hydrogen, and the formation of δ hydrides, lattice defects, hydrogen solution strengthening and Al element diffusion jointly cause the hardness of β phase increase with the increasing hydrogen. © 2008 The Nonferrous Metals Society of China.