Exact interaction mechanism between Bax and Bcl-XL, two key Bcl-2 family proteins, is an interesting and controversial issue. Partial acceptor photobleaching-based quantitative fluorescence resonance energy transfer (FRET) measurement, PbFRET, is a widely used FRET quantification method in living cells. In this report, we implemented pixel-to-pixel PbFRET imaging on a wide-field microscope to map the FRET e±ciency (ET images of single living HepG2 cells co-expressing CFP-Bax and YFP-Bcl-XL. The E value between CFP-Bax and YFP-Bcl-XL was 4.59% in cytosol and 11.31% on mitochondria, conclusively indicating the direct interaction of the two proteins, and the interaction of the two proteins was strong on mitochondria and modest in cytosol.