Ultraviolet (UV) irradiation is a DNA-damaging agent that triggers apoptosis through both the membrane death receptor and mitochondrial apoptotic signaling pathways. Bid, a pro-apoptotic Bcl-2family member, is important in most cell types to apoptosis in response to DNA damage. In this study, a recombinant plasmid, YFP-Bid-CFP, comprised of yellow and cyan fluorescent protein and a full length Bid,was used as a fluorescence resonance energy transfer analysis (FRET) probe. Using the FRET technique based on YFP-Bid-CFP, we found that Bid activation was initiated at 9±1 h after UV irradiation, and the average duration of the activation was 75± 10 min. Bid activation coincided with a collapse of the mitochondrial membrane potential with an average duration of 50±10 min. When cells were pretreated with Z-IETD-fmk(caspase-8 specific inhibitor) the process of Bid activation was completely inhibited, but the apoptosis was only partially affected. Z-DEVD-fmk (caspase-3 inhibitor) and Z-FA-fmk (non asp specific inhibitor) did not block Bid activation. Furthermore, the endogenous Bid activation with or without Z-IETD-fmk in response to UV irradiation was confirmed by Weste blotting. In summary, using the FRET technique, we observed the dynamics of Bid activation during UV-induced apoptosis and found that it was a caspase-8 dependent event.