The purpose of this study is to develop the design equation of a laminar-falling-film-slurry-type (LFFS) photoreactor for the treatment of organic wastewaters (Acid Red 4) by 185 nm vacuum ultra-violet (VUV) related processes. The LFFS photoreactor is one of the most efficient reactor configurations for conducting heterogeneous photocatalytic reactions, particularly for wastewater treatment. The decomposition of Acid Red 4 dye wastewaters by VUV-based photo-oxidation process was studied under various UV light intensities, dosages of H2O2 and TiO2. By the treatment of the LFFS-VUV only process, it was found that the decomposition rates of the dye in aqueous solutions increased with the increasing of VUV light intensity, dosage of TiO2, dosage of H2O2. The apparent potential of OH. generation from the photolysis of used oxidants (i.e. OH. sources, H2O, H2O2, TiO2) to decompose the targeted dye wastewater was investigated and compared.