Stokes polarimeters are most commonly used to measure the state of polarization of optical wave. Dependence of Stokes parameters, degree of polarization on external magnetic field are presented for garnet and groove films on garnet in the transmission and reflection modes. The Stokes parameters S1, S2, S3 of different modes show different tendency and asymmetrically change when the external magnetic field change, while the degree of polarization basically unchange.