The basic theory of a rotatory optical fiber was discussed. The electromagnetic field components, fundamental parameters of the fiber were evaluated as the functions of the argument ε′ which is the rotatory element of the tensor dielectric coefficient matrix ε of the fiber. The calculating results were shown by the curves. In addition, a new expression about the relation between ε′ and bias magnetic field H b was demonstrated, by that the tensor dielectric coefficient of the rotatory optical fiber can be easily eveluated.