Root growth is mainly determined by cell division and subsequent elongation in the root apical area. Components regulating cell division in the root meristematic cells are largely unknown. Previous studies have identified OsRAA1 as a regulator in root development. Yet the function of OsRAA1 at the cellular and molecular level is unclear. Here, we show that OsRAA1 overexpressed transgenic rice showed reduced primary root growth, increased number of cells in metaphase, and reduced number of cells in anaphase, which suggests that OsRAA1 is responsible for limiting root growth by inhibiting the onset of anaphase. The expression of OsRAA1 in fission yeast also induced metaphase arrest, which is consistent with the fact that OsRAA1 functions through a conserved mechanism of cell cycle regulation. Moreover, co-localization assay has shown that OsRAA1 expresses predominantly at spindles during cell division. Yeast two-hybrid and pull-down, as well as bimolecular fluorescent complementation assays all have revealed that OsRAA1 interacts with a rice homolog of RPT4, a component that is involved in ubiquitin pathway. Treating transgenic rice with specific inhibitors of 26S proteasome blocked the degradation of OsRAA1 and increased the number of cells in metaphase. Mutation of a putative ubiquitination-targeting D-box (RGSLDLISL) in OsRAA1 interrupted the destruction of OsRAA1 in transgenic yeast. The results suggest that ubiquitination and proteasomic proteolysis are involved in the OsRAA1 degradation which is essential for the onset of anaphase and that OsRAA1 may modulate root development mediated by the ubiquitin-proteasome pathway as a novel regulatory factor of the cell cycle.