Abstract: The 0.6 mm thick tantalum sheet was lap welded by argon tungsten arc welding, and the microstructure of the welded joint was studied. The results show that the weld was well formed. Oxide layers appeared on the upper surface of the upper tantalum sheet, on the contact surface of the upper and lower tantalum sheets, and on the lower surface of the lower tantalum sheet. The heat affected zone underwent dynamic recrystallization, and the grains of the base metal transformed into an equiaxed shape from the original slender shape and were coarsened under the effects of welding heat. Argon gas can not isolate the air completely, leading to different degrees of oxidation in different areas of the weld. The weld was composed of light colored substances without oxidation or with less oxidation and dark colored substances with a deeper degree of oxidation. Light colored substances had coral-like shapes.