For estimating biological risks from low doses irradiation, BALB/C mice were irradiated to 0, 0.39, 0.55 and 1Gy by X-rays at a dose rate of 0.2Gy/min. The immunocyte cycle and apoptosis were determined by flow cytometry, and the thymus and spleen weight were measured, too. The results showed that the cycle of peripheral blood lymphocyte, thymocyte and spleen lymphocyte were arrested in G,0,—G,1., The apoptosis (expect for spleen lymphocyte) induced by the irradiation increased, whereas the thymus and spleen index declined with increasing dose. The results suggested that low doses X-ray total-body irradiated could result in changes of cycle and apoptosis, and some damages to immunocytes, and the X-ray irradiation affected the organs differently. It might provide experiment basis for radioprotection.