The TiO2 sol formation was monitored by measuring the variation of sol viscosity and particle diameter in the temperature range of 80-120$^{circ}C$ for 2-30h. The starting solution for sol was prepared by mixing the gel which can be produced by the reaction of TiOCl2 and NH4OH solution and hydrogen peroxide solution. The pH value of the resulting solution was 2,4 and slowly increased with time and then neutralized after 40h. It was deduced that TiO2 sol in neutral aqueous solution was formed by the transformation of TiO(OH)(OOH) cluster into TiO2 sol and stabilized by peroxo group existing at surface of TiO2 sol. Average particle diameter of the TiO2 sols aged above 90$^{circ}C$ was 25nm and remained unchanged with time. The crystal structure of sol particles was anatase form