The polarization properties of the output light of two-mode gas lasera ffected by external transverse uniform magnetic field is theoretically investigated. The calculation is given for the polarization paramaters of two-mode He-Ne laser at 632. 8nm with the motion equations deduced for the polarization parameters of two longitudinal modes. The linear polarization output light with high polarization degree is obtained as the strength of the external magnetic field is beyond the threshold value and the experimental results are consistent with the theory.