The sol-gel method was employed to prepare the precursor of zirconium oxide by the reaction of zirconyl chloride octahydrate (ZrOCl2 . 8H(2)O acid EDTA in ammonia. The reaction of ZrOCl2 . 8H(2)O and the ionized complexing agent R(COO-)(n) was achieved through sequential hydrolysis (4ZrOCl(2) . 8H(2)O + 20H2(O) --> [Zr-4(OH)(8)(OH2)(16)](8+) + 8Cl(-)) and polymerization ([Zr-4(OH)(8)(OH2)(16)](8+) + R(COO-)(n) --> [Zr-4(OH)(8-n)(OH2)(16)(-OOC)(n)R](8+) and [Zr-4(OH)(8)(OH2)(16-n)(-OOC)(n)R](8+)). The effect of the reaction conditions, including temperature, the molar ratio of ZrOCl2 . 8H(2)O, the content of NH3, and the agitation speed on the particle size distribution, the average size of particle, and the viscosity of the sol are investigated in detail. A rational reaction mechanism, which accounts for the formation and the growth of the particles and the viscosity of the sol, is proposed to explain the experimental results satisfactorily.