A facile solvothermal technique is developed to prepare a nickel oxide precursor which is subsequently calcined in air at 300°C for 4h to convert it to NiO nanospheres. These nanospheres are assembled from ultrathin mesoporous nanosheets that possess high surface areas. The final product is characterized by various techniques: thermogravimetric analysis, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron spectroscopy, and nitrogen adsorption–desorption method. Such a highly desired structure with a great surface area of 213m2/g is believed to be formed through the Ostwald ripening mechanism. Electrochemical measurements show that our electrode can exhibit a high specific capacitance of 803Fg−1 at a discharge current density of 2Ag−1. The electrode also possesses excellent long-term stability in an alkaline electrolyte. These interesting properties promote it to be a potential supercapacitor electrode material.