Abstract: Effect of C on the conventional mechanical properties and high temperature rupture life of 2.25 CR-1 MO-0.25 V weld metal by submerged arc welding(SAW) was systematically studied, and the key role of carbide on high temperature rupture life was revealed. The heat treatment decarburization process of wire rods could reduce the C content of the welding wire and deposited metal by about 20%, while other alloy elements remained unchanged. With the decrease of C content, 2.25Cr-1Mo-0.25V weld metal by SAW showed characteristics of reduced strength, increased plastic toughness and reduced tendency of tempering embrittlement. Under the test temperature of 540 ℃ and the lasting stress of 210 MPa, the high temperature rupture time of low C weld metal was only 587 h, which was 30% shorter than 939 h of normal weld metal, which could not meet the technical requirements of ≥900 h. The precipitation density and size of carbide in low C welds were significantly lower than that of normal welds. The carbides played a key role of pinning effect to prevent migration of grain boundary sliding and dislocation, which could improve the high temperature rupture life of welds. During the design and product inspection of 2.25Cr-1Mo-0.25V SAW materials, it was necessary to monitor the carbon content of welding wire and weld metal to avoid low C content.