Determination of NO concentration in live cells is essential to evaluate its related cellular functions. In this letter, the concentration of NO in HeLa cells and rat dorsal root ganglion (DRG) neurons are studied by confocal laser scanning microscopy using DAF-2 DA as a fluorescence probe. The results show the fluorescence intensity of NO in HeLa cells is higher than that in DRG neurons, which indicats that the former exhibits higher NO concentration. Furthermore, the experimental conditions for low photobleaching and phototoxicity are optimized.