Saccharomyces cerevisiae cells were irradiated by X-rays with different doses, and then rhodamine123, a kind of fluorescent dye, was applied to measuring ΔΨm (mitochondrial transmembrane potential) by flow cytometry. According to results, the ΔΨm of S'cerevisiae cell obviously decreased compared with that in control group (0 Gy) (p<0.05 or p<0.01), and displayed remarkable correlation with dose after X-rays treatment. The results showed that ΔΨm was declined with the increase of dose before 3 h, while slightly went up at 3 h and the whole was lower than that of the control. After 3 h, ΔΨm maintained the greater level than the control and revealed rising trend with dose and became balanced at 48 h finally. There is a distinct relationship among the ΔΨm and dose, as well as time. These results demonstrate that the change of ΔΨm induced by X-ray irradiation is not simply a drop but presents more complex relations.Cited