Abstract: In this paper,the morphology and behaviour of grain boundary migration at the weld interface in diffusion welding of titanium and its alloys have been investigated by means of high temperature microscopy.The authors find that morphology of β-grain boundary is in close relationship with the state of weld interface prior to phase transfor-mation.In the test pieaes which have not been subjected to pressure prior to phase transformation,a lot of β-grain boundaries perpendicular to the interface will occur as the temperature rises above the phase transformation point.And in the test pieces which have been subjected to pressure for a period of time before phase transformation,grain boundary will cross the interface as soon as it occurs.The migration of the triple point away from the interface and its displacement parallel to the weld interface were the dominant forms of grain boundary motion.The change of energy of the interface was the main driving force of grain boundary migration.Crossing of boundary over the interface will easily occur in the weld interface locally deformed.Therefore,excel-lent diffusion welds can be obtained by the welding procedure in which test pieces are pressed at a temperature below the phase transformation point and then kept at a tem-perature above the phase transformation point for a period of time.