Apoptosis of irradiated MC3T3-E1 cells on day 6–8 after been seeded. MC3T3-E1 cells were exposed to 0, 0.1, 0.5, 1.0 Gy of X-irradiation. Cells on day 6–8 were double stained with annexin-V-FITC and PI and analyzed by flow cytometry. Data were expressed as the mean ± SD of triplicate experiments (n = 3). For each dose, apoptosis cells (including early apoptotic and late apoptotic cells) was gradually increased from day 6 to day 8 and the number of apoptosis cell on day 8 was significantly more than that on day 6 (p < 0.05).