Abstract: High-quality joint of thick TC4 titanium alloy plate was made by narrow-gap tungsten inert gas(TIG) welding with proper gas protection and welding parameters.Metallographic analysis of the resultant joints revealed the rules of grain growth and microstructure characteristics.The variation of the cooling speed resulted in different microstructures in the heataffected zone(HAZ).The tensile test showed that the average tensile strength of the joints reached 90% of that of the base metal, which was attributed to the mechanism of meshing strengthening during narrow-gap TIG welding of thick titanium alloy plate.