In this study, we investigate the relationship between the UVR-shielding rate and the moisture regain of plain woven fabrics. Samples are 8 cotton fabrics and 4 polyester filament fabrics each of which had different porosity. In the case of cotton fabrics, the moisture cause fiber swelling in the wet condition and the porosity decrease with increase of the moisture regain. However, the UVR-shielding rate decrease with the increase of the moisture regain. It is assumed that the result is caused by the increase of the UVR transmitting through a fiber by the fiber swelling. The relationship between the moisture regain and the UVR-shielding rate result in linearity up to 80% of the moisture regain, and besides, significant difference on the inclination of each sample is not found. Therefore, we can estimate the UVR-shielding rate by using porosity under the standard condition and the moisture regain. In the case of polyester filament fabrics, it is found that we can estimate the UVR-shielding rate by the same way as the cotton fabrics. However, the range of the moisture regain in which the UVR-shielding rate can be estimated is smaller than that of cotton fabrics.