In investigating the polarization property and magnetooptic effect of single-mode fiber, we need to consider not only its Faraday effect but also its birefringence. In this paper, the theory of the magnetooptic effect of the single-mode fiber is described with the Jones matrix. The Jones matrix expressing the birefring ence and magnetooptic effect of the single-mode fiber is derived. And the relation between the output light intensity and Faraday rotation angle of current sensor system is presented using Mueller matrix.