The effects of water vapor in hydrogen gas on the friction coefficient and wear loss of Co-based alloy Stellite 6B. The friction coefficient decreased with the humidity. In the extremely dry 0.22% condition, the friction coefficient gradually increased with cycle number. Wear loss decreased with humidity. In the extremely dry 0.22% condition, severe adhesion was observed on the stationary specimen and severe abrasive wear was observed on the moving specimen. In the humid conditions over 2.4%, no severe adhesion was observed. In 100% humid condition, the worn surface was very smooth and wear debris were relatively small. These results indicated that severe adhesion was prevented by the presence of water vapor. This is an abstract of a paper presented at the World Tribology Congress (Kyoto, Japan 9/6-11/2009).