Nickel nanoparticles are prepared successfully through reducing nickel chloride by hydrazine hydrate and are tested as supercapacitor electrode material for the first time. The as-prepared nickel nanoparticles are characterized intensively by a variety of means such as SEM, TEM, XRD and XPS. TEM observations and XRD analysis demonstrated that the size of nickel nanoparticles is about 12 nm. XPS analyses indicate that the surface nickel atoms can react easily with O-2 and water in the atmosphere to form nickel oxide/hydroxide species. As evidenced by electrochemical measurements, these surface nickel oxide/hydroxide species can generate substantial pseudocapacitance, reaching up to 416.6 Fg(-1) for nickel nanoparticles, which is higher than most carbon electrode materials reported in the literatures. This kind of surface metal oxides/hydroxides that generate pseudocapacitance may also occur on other metal nanoparticles except nickel nanoparticles, which provides a new approach to searching for electrode materials with even higher capacitance. (C) 2012 Elsevier ay. All rights reserved.