Abstract: The polarization observations are carried out towards OH masers in W33B with the ²Ⅱ_3/2, J = 3/2, F=1→1, and F = 2→2 transitions. The magnetic strength of 5 mGis inferred by the Zeeman splitting between right and left circular polarization spectra. The hydrogen density of the protostellar matter, n(H₂) =3×10⁷cm^-3, in the active massing region is estimated by considering a spherical maser with the fully saturation and the homogeneous pumping.