The relationship between the size of ZnO particles in porous semiconductor electrodes and their photovoltaic performance for dye-sensitized solar cells (DSSCs) was investigated. The DSSCs employing 40 nm ZnO particles showed the best power conversion efficiency among those based on particles in the size range 20 nm to 2 mu m. The cell performance drastically changed further upon TiCl4 post-treatment of the ZnO layer, and a treatment time of more than 20 min was found necessary to improve the performance.