Nanoparticles of cobalt-nickel spinel oxide (cobaltite) were prepared by chemical co-precipitation. First, mixed metal oxalates were co-precipitated using nickel and cobalt chloride aqueous solutions, and oxalic as a precipitant. The mixed oxalate precursors were ignited at 350 oC then calcites at 600 oC for 6 h to produce mixed metal oxides and/or cobaltite. The produced nanoparticles were characterized by thermogravimetric– differential thermal analysis (TGA/DTA), Fourier transform infrared spectroscopy (FTIR), X – ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM). The FTIR spectra of the mixed oxides nanoparticles show two absorption band in the region 660-665 and 555-562 cm-1 corresponding to the metal-oxygen stretching from tetrahedral and octahedral sites respectively, which are characteristics of cobaltite. The catalytic efficiency of the prepared nanoparticles on the photo-degradation of methylene blue (MB) under UV light irradiation was investigated where it was found that the dye is 69% degraded after 120 min in presence of the catalyst compared to 21% degradation without catalyst during the same time.