Based on the Huygens-Fresnel Principle and the unified theory of coherence and polarization of partailly coherent beams, the degree of polarization of partially coherent vortex beams transmitted through the circular aperture has been investigated. It is found that the degree of polarization will change after the beam transmitted through the circular aperture, and the degree of polarization of the diffractive beam is related with the coherence of source, the topological charges of the source, and the size of the circular aperture.