Nanoplates-composed ball-flower-like Bi2WO6 (PB-Bi2WO6) was synthesized by a hydrothermal method without any organic precursor and its performance in photocatalytic reduction of CO2 was investigated in a continuous-flow reaction system under visible light irradiation (420 nm < lambda <620 nm). CO was detected as the main product of this photocatalytic process and H2O was found to suppress the conversion of CO2 to CO due to its competitive absorption with CO2 on the medium strength basic sites of Bi2WO6. PB-Bi2WO6 annealed at 550 degrees C showed superior CO yield in the condition with little water vapor. It might be attributed to the enhanced crystallinity, significantly decreased recombination rate of photo-generated electrons and holes and more stable basic sites for strengthened CO2 adsorption, according to characterization results by XRD, SEM, UV-vis SRS, PL and CO2-TPD. However, comparing with PB-Bi2WO6, the negative effect of H2O was even more prominent on the annealed sample because of the reduced surface area. Yield decrease was observed during the irradiation time due to the adsorption of intermediates generated but fortunately washing with deionized water was found to be an effective way to renew the catalyst. (C) 2014 Elsevier B.V. All rights reserved.