Completely reversed plane bending fatigue tests were carried out using small size plate specimens made of Al-killed low carbon steel in two environments. One was uncontrolled room atmosphere and another was dry atmosphere whose relative humidity was reduced to about 1×10-4. The effect of the reduction of humidity was clearly observed and the fatigue life as well as the fatigue limit were largely improved in the dry atmosphere. This improvement was due to the delay of the initiation of microcracks on the specimen surface. Moreover, it was observed that in both environments the microcracks originated in the close neighborhood of boundaries of several surface grains having the optimum orientation for slip and its larger portion buried below the surface.