The effects of source cross polarization on the polarization degree of radiation beam were investigated. Formulas expressing the spectral degrees of coherence, of polarization and the degree of cross polarization of a Gaussian-Schell-model source were analytically derived. From these formulas it was demonstrated that sources with the same set of degrees of coherence and polarization may have different degrees of cross polarization. The spectral degrees of polarization as a function of the distance from the source were calculated for coherent and incoherent sources respectively. The plots show that the correlation coefficients between the two mutually orthogonal components of sources may evidently affect the longitudinal evolution and the asymptotic values of the polarization degree.