In this study, the photocatalytic degradation of acetaldehyde into CO2 with coated photocatalyst on the flat substrate was conducted. To obtain the detailed characteristics of the reaction the Lanmuir-Hinshelwood kinetics was applied. Modelling of the reaction was successfully achieved with the aid of constants(kL-H, K) extrapolated from the experiments. Reaction rates increased with increased coated area under the same initial concentrations and light intensities. The order of rate, however, changed first to zero with increased light intensity and the same coated area. Those two results brought an issue that at certain light intensity a less-coated area may result in a higher rate, which was finally identified experimentally in this study. The light intensity and the coated area are the most critical factors in photocatalysis, the optimal combination of which can show the synergy.