Abstract: The effect of relative humidity on the static friction coefficients of four frictional pairs, namely, metal  metal, metal  graphite, metal  TiN coating, and metal  TiN coating pairs, was examined on a friction and wear tester made in our laboratory. It has been found that the relative humidity has no influence on the static friction coefficients of metal  graphite and metal  WC coating pairs, but has influence on those of metal  metal and metal  TiN coating pairs. The fractal contact model was used to calculate the additional static friction coefficient caused by water film meniscus. As the results, the calculated additional static friction coefficients conform well to the tested ones, but the calculated values are slightly larger than the tested ones.