Nanosized NiO2 particles with an average diameter of 15 nm are prepared by treating of Ni(NO3)2 · 6H2O with an aqueous solution of KClO in the presence of Triton® X-100. This black fine powder of nickel peroxide was characterized by XRD diffraction, energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). The as-prepared NiO2 can be easily transformed to nanosized NiO merely by washing it with acetone. The obtained NiO has an average diameter of 40 nm and was characterized by the same means used for NiO2. The nanoparticles of NiO2 and NiO were obtained in high yields and purities.