The wear behavior of two types of Si_3N_4 exposed to high and low humidity was examined at various sliding speeds, using bearing steel as disk material under pin-on-disk type sliding conditions. Higher wear rates were obtained at a high humidity than at a low humidity. As the sliding speed was increased, the wear rates were decreased and the effect of humidity on the wear rates of Si_3N_4 was reduced. The result that the Si_3N_4 pin showed higher wear rate under the high humidity condition was explained by the decrease in microhardness of Si_3N_4 due to the chemisorbed moisture on the pin and plowing action by the hard particles of Fe_2O_3 from the disk. An increase in the sliding speed is supposed to reduce the effect of humidity on the wear rate of Si_3N_4 by raising the average temperature of the disk surface and the local temperature at pin-disk contact point.