The effects of trace yttrium oxidation on the properties and the microstructures of 9Cr-1Mo steel welds for super critical generation units and USC units have been studied. Yttrium has been added into weld metal by coating transition (e. g. molten droplet method). The yttrium content for optimized properties and structures has been determined. Comparison tests including room temperature and high temperature tensile tests, serials impact test, metallographic analysis, diffusion hydrogen measuring, phase transformation point, SEM, XDR etc. have been performed on the welds with and without yttrium. It is found that second phase particles make an important contribution to improving the properties of the weld metal, especially low temperature impact performance