Abstract: To study the hardfacing process of reactor vessel internals with cobalt alloys, the cobalt-base alloy Stellite 6 was deposited on the surface of 304H stainless steel by gas tungsten arc welding (GTAW) process using different welding parameters, after which a series of tests were conducted on the deposited metal to analyze its chemical composition, cross-section microhardness, Rockwell hardness, intergranular corrosion and the microstructure, etc. Results show that both the electrical parameters and the weld thermal cycle have obvious effects on the chemical composition, hardness and microstructure of the deposited metal; under the welding parameters of preheat temperature at 176℃, welding current in 145-190 A and welding speed in 8-12 cm/min, the acceptance rate of deposited weld gets up to 97%; the risk of cracking of the deposited metal can be further prevented by depositing a 3 mm transition layer on the surface of stainless steel 304H with welding wire ER308L, prior to the final hardfacing process with cobalt-base alloy Stellite 6 at the preheat and interpass temperature of 400℃.