As the effect of moisture regain is very large on the mechanical properties of textile fibers containing hydrophilic groups, it is quite interesting to study on them. In this paper, the relations between the dynamic properties and regain of some textile fibers (Fortisan, high tenacity rayon, Bemberg, viscose rayon wool, nylon, acetate), which have got affinity to water absorption are studied. The stretched vibrometer system and the direct measuring method were combined as a measuring method, and a constant frequency 106cps is used. The humidity control was performed by mixing dry air with damped air and all the samples were tested at 20°C. The constant tension applied to the fiber is 0.2gr/denier. Until about 10% of regain the dynamic modulus of regenerated cellulose fiber decreased linearly with the increase of regain, but at higher regain region the decrease was rapid. Loss tangent, tan δ increased slowly in the region where the linearity of dynamic modulus exists and in the high regain region it increased rapidly. The difference of tan δ with the fine structure of various regenerated cellulose fiber is not much recognized and is of the same order. The change of dynamic modulus due to regain depends on the orientation and lateral order of cellulose fiber in the low order region. The dynamic properties of wool and acetate due to water adsorption showed gradual change in the low regain region, but in case of 6-nylon, the change of tan δ was large and could not be expressed by a linear relation. These difference were described by the flexibility of the chain molecule of the fiber and relaxation time of molecular segment of the large side groups. The region where the relation of the dynamic modulus and tan δ with the regain was linear has been determined and the experimental results are shown in Table 2. Some dynamic loss which is the product of tan δ and dynamic modulus showed maximum value and some did not. Bemberg, high tenacity rayon, viscose rayon, and nylon belong to the former case and Fortisan, acetate and wool belong to the latter. The result which was obtained by Quistwater for the dynamic properties of 6.6 nylon showed the same tendency of the effect of regain for 6-nylon. A double network structure can be conceived for cellulose fiber and it is thought that the main network structure is sustained in the wet state and the sub-network structure is broken by water adsorption. It is assumed that the stress is propagated in the sub-network structure mainly by hydrogen bond. The relation of modulus and hydrogen bond according to Nissan's formula, n∝E3 (n is the number of hydrogen bond in unit volume and E is the modulus) was verified. With the elementary formula i.e., the breaking number of hydrogen bond with the addition of water is proportional to the number of unbroken bonds, the process of breaking of more than one hydrogen bond by one water molecule as an autocatalic effect was considered and the formula was calculated. In the low regain region the elementary relation was established in the case of cellophane, which possesses low orientation. But Fortisan, high tenacity rayon, viscose rayon and others possessing high orientation showed a deviation from this elementary relation in the low regain region. In the low regain region the breaking of two hydrogen bonds is done by one water molecule, and in the medium regain region, viscose rayon showed the maximum value, where 6 hydrogen bonds were broken. High tenacity rayon showed rapid increase of breaking number of about ten bonds in the high regain region. In the case of Bemberg the breaking was not so rapid as high tenacity rayon but was rather slow. The autocatalic effect was observed for cellophane in high regain region.