CuO/ZnO nanocomposite has been synthesized using Melissa Officinalis L. leaf extract as a mild, renewable and non-toxic reducing agent and efficient stabilizer in the absence of hazardous and toxic materials. SEM, EDS, elemental mapping, TEM and XRD have been used for the characterization of CuO/ZnO nanocomposite. CuO nanoparticles (NPs) with sizes in the 10-20 nm range have been formed, as shown by TEM micrograph. Furthermore, excellent catalytic activity in the degradation of 4-nitrophenol (4-NP) and Rhodamine B (RhB) in water at ambient temperature has been shown by CuO/ZnO nanocomposite.