Cell cycle regulation is closely linked to plant growth and development. The plant cell cycle responds rapidly to stimuli from both the internal and external environment. We present a model of partially synchronized root tips of Arabidopsis to study the effects of UV-B radiation on the G1/S transition in the cell cycle. Analysis of the fluorescence image under micros copy indicated that G1/S arrest was induc ed by UV-B radiation. The expression of two factors of G1/S transition, Histone H4 and E2Fa, was repressed under UV-B radiation, whereas the negative factor of G1/S transition, KRP2, was upregulated by UV-B radiation. UV-B radiation during the cell cycle induced accumulation of cyclobutane pyrimidine dimers. All these results suggested that UV-B induced plant growth inhibition may be mediated by cell cycle regulation and DNA damage.