TiO 2 nanorods arrays (NRAs) have been considered as very promising photoanode materials in photoelectrochemical (PEC) cells. However, the performance of TiO 2 NRAs still requires substantial improvement in order to reach the goal of practical applications. Annealing treatment of TiO 2 NRAs can help to improve the PEC performance, but the mechanism is still not yet fully understood. In this work, we systematically investigated the optical and electronic properties, as well as the PEC performance of the thermally treated rutile singlecrystalline TiO 2 NRAs. Surprisingly, we recorded a maximum photocurrent density of 1.38 mA/cm 2 at 1.3 V versus reversible hydrogen electrode for TiO 2 NRAs annealed in O 2 , which is about 28 times higher than that of the pristine TiO 2 NRAs. We further revealed that the surface adsorbed Cl ions largely suppress the photoresponse of the TiO 2 NRAs as they serve as recombination centers and block the adsorption of water molecules to the surface of TiO 2 NRAs. The enhancement in photocurrent after annealing in O 2 is due to the desorption of the Cl ions, filling of the surface V o , expansion of the depletion layer, and increase of the grain size. Our results shed light on the effect of annealing on the PEC performance of TiO 2 NRAs and offer guidance for annealing of other semiconductor materials.