A two-step approach is proposed to prepare high-performance NiO electrode material. First, the scroll-like α-Ni(OH)2 is prepared by hydrothermal reaction via a self-assembly growth process using guanidine hydrochloride as precipitant. Second, the precursor Ni(OH)2 is converted to NiO by calcination. The resultants of hierarchical mesoporous NiO ultrathin nanosheets are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) patterns and N2 adsorption and desorption. The electrochemical properties of the samples are evaluated though cyclic voltammetry (CV), charge–discharge and electrochemical impedance spectroscopy (EIS) in 6.0 M KOH electrolytic solution. The results show that the typical hierarchical mesoporous NiO ultrathin nanosheets exhibits a very large specific capacitance of 1060 F g−1 at 1 A g−1, an outstanding cyclic stability with a capacitance retention of 91% after 5000 cycles of charge–discharge and a low resistance.