Yttrium oxide hydroxide nitrate (Y4O(OH)9NO3), which was prepared by polyvinyl alcoholassisted hydrothermal method from the Y(NO3)3 and KOH starting solution, was characterized by XRD and FE-SEM. The results revealed that pH value of the starting solution is an important factor which determines the morphology and size of the products. The obtained products varied from lamina microprisms to circular nanorods when the pH value increased from 7.0 to 8.0. In the high pH value above 8.0, the size of nanorods decreased with increasing pH value. A primary mechanism was proposed to explain the evolution of morphology and size of Y4O(OH)9NO3 products prepared under various pH values.