Abstract: The welded joint microstructure of 12 mm thick Ti6321, welded by TIG, has been analyzed by using OM and TEM, and the rule of the microstructure of different joint area has been researched. The results show that the microstructure of seam is columnar grains, which contain course α. The coarse α is formed because the effect of the latter weld seam on prior weld seam. The acicular α' is formed for a short residence time for latter weld seam at high temperature; HAZ comprised transition zone, fine grain zone and coarse grain zone, the grains in HAZ are equiaxed but with different sizes and forms. Transition zone is consisted of equiaxed α, rod α' and remained β grain. The grain zone is consisted of rod α' and remained β grain; coarse grain zone is consisted of acicular α' and remained β grain.