Abstract: Fine grain titanium alloy is used extensively in aerospace and aircraft because of its excellent comprehensive properties and outstanding machinability.The evolution of phase, the effect of welding heat input on the grain growth in the coarse grain heat-affected zone(CGHAZ)and mechanical properties of welded joint during gas tungsten arc welding of ultra fine grain Ti-6Al-4V with lamellar microstructure were investigated.Measurement results indicate the severe grain growth coarsening in CGHAZ undergoing welding thermal process.In the CGHAZ, phase transformation during welding process resulted in a microstructure containing α'martensite. Small crossed α'clusters transited into coarse α'beames through the whole grain in the coarse grain heat-affected zone with increasing the heat input.Hardness measurements showed there were softened zone in heat-affected zone, and width of this zone raise with increasing the heat input.However, tensile strenght of welded joint reduced with increasing the heat input.Tensile fractrues located in coarse grain zone present character of quai-cleavage crack with higher heat input. Mass tear ridges had be found on fracture surface, and a large number of dimples existed in the surface of intercrystalline crack.