Ultrathin TiO2 was grown on hematite surface by atomic layer deposition (ALD). Obvious photoelectrochemical water oxidation performance improvement was observed for samples treated with as few as a single cycle of TiO2 deposition. Up to 100 mV cathodic shift of the turn on potential was measured on samples treated by 20-cycle ALD TiO2. Photocurrent improvement was also measured on samples treated by ALD TiO2. Systematic studies ruled out possibilities that the improvement was due to electrocatalytic or bulk doping effects. It was shown that the surface treatment led to better charge separation, less surface charge recombination and, hence, greater photovoltage by hematite. The facile surface treatment by ultrathin TiO2 may find broad applications in the development of stable and high-performance photoelectrodes.