A simple method has been developed to synthesize single-phase TiO2 nanocrystals with uniform morphology and size at room temperature. The morphology and crystal phase of the products can be easily controlled. A possible control mechanism is proposed. The photocatalytic performances of the products were evaluated by the photodegradation of methyl orange in water. The experimental results indicate that the dye degradation efficiencies of the as-prepared rutile TiO2 nanocrystals are almost twice that of the commercial photocatalyst P25.