The formation rate, crystal phase, and primary and secondary particle sizes of hydrous zirconia particles produced under the hydrolysis conditions of various ZrOCl2 concentrations (0.1-0.4 mol/dm(3)) and reaction temperatures (343-373 K) were measured to investigate the effects of ZrOCl2 concentration and reaction temperature on the formation process. Chemical analyses and X-ray diffraction measurements revealed that hydrous zirconia particles synthesized under all of the hydrolysis conditions were similar to those observed in monoclinic crystalline zirconia, except that the crystal structure changed as the chlorine content increased under the synthesizing conditions of 0.4 mol/dm(3) and less than or equal to363 K. The primary particle size of monoclinic hydrous zirconia particles decreased as the ZrOCl2 concentration increased and was independent of the reaction temperature. The secondary particle size of hydrous zirconia particles synthesized at ZrOCl2 concentrations of 0.1, 0.2, and 0.4 mol/dm(3) increased monotonously, first increased and then decreased, and decreased monotonously as the reaction temperature decreased, respectively. The rate constants (k) of the hydrous zirconia particles were determined experimentally by applying Avrami-Erofeev's equation. The determined k value decreased as the ZrOCl2 concentration increased and the reaction temperature decreased. From Arrhenius plots of k, the activation energies for hydrous zirconia particles in the monoclinic phase and containing high chlorine contents were determined to be 387-396 and 249 KJ/mol, respectively. The nucleation and crystal-growth mechanisms of primary particles and the formation mechanism of secondary particles formed by hard aggregations among primary particles were determined based on the present experimental results.