Rainwater utilization has great potentials for recovering hydrological cycles, for buffering extreme run-off situations in the watercourses, and for reducing the costs for water supply in urban area. However, little information is available on the water qualities and runoff properties of collected rainwater, which is critical for efficient design and maintenance strategies of rainwater harvesting facilities. In this study, characteristics of rainwater collected in a rainwater harvesting system were examined in terms of pollutant concentrations. Based on these results, TiO2 photocatalysts were applied on the roof surface as a novel coating material to improve the quality of collected rainwater. The analysis of rainwater samples shows that the contamination by turbidity, conductivity, and color occurs during the catchment process, which may result in troubles for further use. The roof coating technique developed in this study appeared to be effective in increasing the wash-off of pollutants and improving the water quality in roof runoff because of its increased hydrophililicy as well as photocatalytic reactivity. Further research is required on trace pollutants such as endocrine disruptors to estimate and minimize health risk in rainwater harvesting.