TiO2 nanocrystallines with a size of 3-7 nm have been synthesized by the sol-gel method with excessive water at a low temperature of about 50 degrees C. The effects of amount of water and pH on the formation of sols and powders containing TiO2 nanocrystallines were investigated by X-ray powder diffraction (XRD), transmission electron microscopy, and high-resolution transmission electron microscopy. The results showed that a pure anatase phase was formed at pH 1, in contrast to the formation of particles with a mixture of anatase and brookite phases at pH 2-7. Moreover, at pH 1, the intensity of the XRD peaks and the crystal size increased with increasing water content when the water ratio increased from 40 to 100. Whereas the crystal intensity remained invariant, the crystal size still increased when the water ratio increased from 100 to 400. The photocatalytic activity was also investigated. It was shown that when the water ratio was below 100, the photocatalytic activity increased with increasing water content. Further increase of the water content decreased the photocatalytic activity. The photocatalytic activity decreased with increasing pH. All samples with pH = 1 showed better photocatalytic activity than Degussa P25. Mechanisms for the formation of the TiO2 nanocrystalline particles at such a low temperature were also discussed.