The nanocrystalline TiO_2 powder were prepared by improved sol-gel method at low temperature under ambient pressure. The prepared TiO_2 powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM). It is found that the yield of anatase TiO_2 significantly increases with the reduction of pH by increasing the nucleation rate of anatase. There is an optimum amount of water in sol for the formation of anatase by combining two effects on the concentration of Ti(OH)_4 in gels. Increasing in reaction temperature also benefit to the nucleation of anatase of TiO_2. The nanocrystalline TiO_2 powder were prepared by improved sol-gel method at low temperature under ambient pressure. The prepared TiO_2 powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM). It is found that the yield of anatase TiO_2 significantly increases with the reduction of pH by increasing the nucleation rate of anatase. There is an optimum amount of water in sol for the formation of anatase by combining two effects on the concentration of Ti (OH) _4 in gels. Increasing in reaction temperature also benefit to the nucleation of anatase of TiO_2.