After whole head irradiation (WHI) of mice with ,12,C,6+, ions (80MeV/u), changes of cycle distribution of spleen cells were studied, in an attempt to provide basic data for heavy ion radiotherapy and space radiation protection. The heads of the BALB/c mice were irradiated with 0, 0.5, 1, 2, 4 or 10Gy by the ,12,C,6+, ions. The cell cycle distribution was analyzed by flow cytometry. Thirty-six hours after the irradiation, the S phase cells were reduced significantly (,p,<,0.05) with increasing doses. The 0.5Gy, 4Gy and 10Gy WHI groups showed evident arrest in G,1,/G,0, phase (,p,<,0.05), but the 1Gy and 2Gy WHI groups had no significant arrest (,p,>,0.05). G,2,/M phase of the 0.5Gy WHI group was reduced markedly (,p,<,0.01), while the other groups showed obvious arrest (,p,<,0.05). After WHI of mice with the heavy ions, spleen cell cycle distribution of mice changed significantly.