Hydrophilic photo luminescent semiconductor quantum dots (QDs) are novel nanometer-size probes, which may have potential using in bio-imaging for biological objects. In this work, the photo-stability of these QDs in two kinds of living cells was studied, compared with conventional biological probes such as fluorescein isothiocyanate (FITC) and green fluorescence protein (GFP). It was found that the concentration of QDs in living cells is the dominant factor for its photo-stability in biological environment. When the concentration of the intracellular QDs was high, the QDs show good photo-stability that is much better than the organic fluorescent probes. However when its concentration was low, the QDs also can be photo-bleached quickly. Thus the reaching of the certain concentration level is the critical condition for QDs in the application of bio-imaging.