Understanding the mechanism of cell cycle control in plants is important for agricultural technology enabling enhancement of growth rate and then crop yield. Progression of the cell cycle requires the cell cycle-dependent appearance of cell cycle regulatory proteins such as cyclin-dependent kinases (CDKs) and cyclins. These regulatory proteins are controlled at several levels, including expression,phosphorylation, and interaction with other regulatory proteins. Recently, it has become clear that the controlled and timed destruction of the proteins plays an essential role in cell cycle regulation and the 26S proteasome is involved in the destruction. For example, Cyc (cyclin) A, CycB, CycD, and B-type CDK are degraded by 26S proteasome in a cell cycle-dependent manner. In this review, recent advances in our understanding of cell cycle regulation through ubiquitin/proteasome-mediated proteolysis in plants are described.