9% Ni steel was electron beam welded and the impact toughness of its weld metal was investigated. The improvement of weld metal toughness was studied by an increase of Ni content in the weld metal and its mechanism was discussed. The weld metal impact toughness was as low as 25-55 J at -196 degreesC. However, it increased remarkably by an increase of Ni and showed 118 J at the same temperature when Ni content was 16.6%. As Ni lowers the martensite start temperature, martensite formed in weld metal with a high Ni content has more chance of auto-tempering because of its slow cooling rate at a low temperature range. Therefore, the improvement is believed to be due to the formation of auto-tempered martensite and retained austenite during the cooling cycle after welding