The transfer of water vapor from the skin through clothing is an important factor in human comfort. This factor is resistance to the diffusion of water vapor through textiles. The resistance of woven fabrics depends not only on the kind of fiber, yarn number, thickness, cloth count, and its cloth structure, but also on the pore diameter and the open free area. The rate of water vapor transmission of woven fabrics increases in proportion to the pore diameter, the open free area, and the relative open free area(the ratio of the open free area to the thickness). Consequently this paper was shown the methods of the calculation of the pore diameter and the open free area from the characteristics of worsted fabrics made of ring spun yarns, and was evaluated the water vapor permeabilities of these fabrics by changes of the pore diameter and the open free area.