Abstract: Apoptosis is an important cellular event that plays a key role in pathogeny and therapy of many diseases. Upon stimulation by various apoptotic inducers, Bax undergoes conformational changes, oligomerization, translocation from cytosol to mitochondria. Despite recent progress in the study of Bax during apoptosis , it remains unclear how Bax translocates to mitochondria and then induces the release of cytochrome c and other substances. In order to further study the biological action of Bax during apoptosis , especially under physiological condition of living cell , the distribution of Bax in single living cell during photodynamic therapy-induced apoptosis was primarily investigated by the use of fluorescence microscopy. The result showed that Bax equably distributed in cells without PDT treatment, thus the activation of Bax started 15 minutes after PDT treatment . This study suggests that it is possible to use fluorescence techniques to dynamically detect the mechanisms of Bax translocation and oligomerization in living cells.