A prediction equation of the steady state water vapor transfer resistance of woven fabrics was obtained based on a simple water vapor transfer model through a simplified fabric structural unit. It was assumed that the volume fraction of air space within fabrics is larger than the fiber volume fraction, and that the water vapor resistance through fibers, even for hygroscopic fibers such as cotton, is much larger than that of air. Therefore, the effects of both the intra-yarn and inter-yarn interstices were taken into consideration, but the effect of the passage through hygroscopic fibers was not included in the transfer model. As for the fabric structure parameters used for the calculation, the simpler parameters of fabric thickness, weave density, yarn count, and fiber density were selected from the viewpoint of the application of fabric performance design. The validity of the prediction equation was verified by comparing the calculations and the experimental data of water vapor resistances measured for 40 woven fabric samples. This result also suggests that the effect of the air space within fabrics on the water vapor resistance of fabrics under the steady state condition is much larger than that of the passage within hygroscopic fibers.