Peroxynitrite is a potent oxidizing and nitrating agent which has detrimental effects on cells by altering the structure and function of biomolecules present within. A fluorescent probe rhodamine B phenyl hydrazide (RBPH) has been proposed for peroxynitrite (ONOO(-)) imaging in MCF-7 cells based on its oxidation property, which converts RBPH to pink colored and highly fluorescent rhodamine B. The fluorescence emission intensity of the rhodamine B produced in the above process is linearly related to the concentration of peroxynitrite. The method obeys Beer's law in the concentration range 2-20 nM and the detection limit has been found to be 1.4 nM. The possible reaction mechanism of peroxynitrite with RBPH to produce rhodamine B has been discussed with spectroscopic evidence. The Probe is selective to the peroxynitrite in the pH range 6-8 which is near physiological pH. Fluorescence microscopic studies suggest that the probe is cell permeable and hence peroxynitrite was imaged in MCF-7 cells.