To investigate the radioprotective effect of vitamin D against irradiation injury, the mice exposed to ,60,Co γ-rays at 6 Gy was treated with preparation of vitamin D(Alfacalcidol Soft Capsules). Cell cycle and apoptosis was analyzed by flow cytometry (FCM) following staining of cells with propidium iodide (PI). Peripheral blood cell counts were analyzed by autoanalyzer. It has been found that vitamin D significantly increases white blood cell (WBC) counts, decreases bone marrow PEC micronucleus rate. FCM analysis shows that compared with damaged group,G2 and S phases of bone marrow cells in vitamin D protection group increases significantly at 24 h after whole body irradiation, whereas G1 phase cells decrease at the same times. So vitamin D might be a new radioprotection agent and it should be deserved further study.