Artificial photosynthetic systems are promising approaches to convert sunlight into chemical fuels. The most popular strategy is photoelectrochemical (PEC) water splitting to produce hydrogen sustainably. While a multitude of semiconductors have been investigated for potential use in PEC water splitting, the most critical issues are still the development of semiconductors with high photon to charge-carrier efficiency in the visible light region and long-term durability in an aqueous environment. WO3 is an n-type semiconductor, and one of the most promising materials ,however, the relatively low activity in the visible light contributes to the poor overall efficiency for PEC water oxidation. Here we report that a synthesis of single crystalline WO3 on a fluorine-doped tin oxide. The WO3 has a typical microplate shape with perpendicularly oriented to the FTO substrate. The scanning electron microscopy image showed high uniformity of the WO3 microplates and precisely controlled to the average length and thickness only change the concentration of both H2WO4 and oxalic acid. The X-ray diffraction patterns were in good agreement with that of monoclinic WO3 and the relative intensity of the (002) reflection of WO3 microplates electrode is large compared to the reference peak, indicating the coexistence of an oxygen-deficient structure. The PEC performance of the WO3 microplate and nanoparticle electrodes have been studied using linear sweep voltammetry (LSV) in 0.1 M Na2SO4 (pH 7). All electrode samples successfully showed anodic photocurrents, and the microplate with a thickness of 470 nm (MP1) generated 1.9 mA/cm2 at 0.6 V (vs Ag/AgCl) under UV-vis illumination. This result indicate that the thickness of the MP1 was in good agreement with the maximum hole diffusion length, indicating an optimized morphology for effective photogenerated charge separation. Note that the MP1 electrode was better photocurrent under visible light irradiation (λ > 425 nm) compared to other WO3 electrodes. These enhanced levels of activity mainly originated from the morphology of the single-crystalline domains with an optimum thickness, and the optimized oxygen deficiency of WO3 due to the use of an appropriate annealing procedure.