Metalorganic chemical vapor deposition (MOCVD) growth assisted by UV light illumination for nominally pure ZnS epitaxial films has been carried out in a low-pressure system. The effects of UV light (>3 eV) have been investigated using low-temperature photoluminescence (PL) measurement. As a result, an increase in Na-related emission lines and a reduction of deep level emission have been observed in the PL spectra. This suggests that the improvement of the crystallinity and introduction of the Na acceptor-impurity substituted on Zn lattice site occured during the epitaxial growth under UV light illumination. We consider that these effects arise from the migration enhancement of adsorbed Zn atoms and/or Zn vacancies by irradiation during the growth, which will lead to a significant change of Zn and S compositional ratio. Finally, we found it profitable to apply these UV light illumination effect to the Na acceptor doping, and thus carried out the photo-assisted Na doping in the same system. A typical PL spectral feature of p-type ZnS films can be successfully obtained.