AbstractWhen an isotropic optical fiber is twisted, its permittivity becomes anisotropic. In case of a circular fiber, the polarization plane rotates because propagation constants of left‐handed and right‐handed circularly polarized waves are different. In case of an elliptic fiber, the left‐handed and right‐handed circularly polarized waves couple with each other and its polarization characteristics are complicated. The polarization characteristics of the fiber with small twist can be approximated to those of the elliptical fiber and approach those of the circular fiber as the twist angle increases. The effect of the cross‐sectional deformation on the polarization characteristics may be eliminated by applying a large twist on the fiber. The polarization characteristic curves have 90° periodicity with regard to the incident polarization angle. The circularly polarized wave is generated at some incident polarization angle when the ellipticity and twist angle are optimum. These findings coincide with the experimental results.