A flame reactor embedded with a constant tungsten wire feeding system to prepare one-dimensional (1-D) nanostructured tungsten oxide thin film for photoelectrochemical (PEC) water splitting was developed. Photoactive vertically-aligned nanowire-based WO3 thin films could be obtained with a controlled thickness via a flame vapor deposition process followed by air-annealing. The PEC performances of WO3 photoelectrodes for different thin film thicknesses were examined. The optimum thickness of WO3 thin film was found to be about 7.2 m for PEC water splitting based on incident photon-to-current efficiency plots and I-V curves. The WO3 prepared with optimum thickness showed better PEC performance than those of recently reported nanostructured WO3 photoanodes. (c) 2015 American Institute of Chemical Engineers AIChE J, 62: 421-428, 2016