The primary and secondary particle sizes of monoclinic hydrous-ZrO2 particles produced by the hydrolysis of various ZrOCl2 solutions, with and without the addition of NaCl, CaCl2 or AlCl3, were measured using X-ray diffraction and transmission electron microscopy in order to clarify the formation mechanisms of primary and secondary particles. The primary particle size of hydrous ZrO2, under a constant ZrOCl2 concentration, decreased monotonously with increasing Cl--ion concentration. On the contrary, the secondary particle size increased monotonously with increasing Cl--ion concentration. The present experimental results revealed that the primary and secondary particle sizes of hydrous ZrO2 are controlled primarily by the concentrations of H+ and Cl- ions produced during hydrolysis, and are independent of the type of added metal ions. The formation mechanisms of the primary and secondary particles of hydrous ZrO2 were determined on the basis of the present experimental results.