Constructing nanostructures with desirable morphology and size is a critical issue for pursuing high performance electrode materials. Ultrathin Co3O4 nanosheet arrays, which are composed of well aligned uniform long-range (~5 μm in length) and thin (~10 nm in thickness) nanosheets, with reasonable mass loading on Ni foam are prepared by a two-step hydrothermal reaction. As a supercapacitor electrode, a superior specific capacitance (~1782 F g(-1)) is obtained at current density of 1.8 A g(-1) (5 mA cm(-2)), much larger than that of the thicker nanostrucutures (~300 F g(-1)). The ultrathin nanosheet arrays electrode exhibits good rate capabilities, maintaining 51% of the initial capacity at current density of 30 mA cm(-2), and excellent long-term stability, remaining >90% of capacitance after 2000 cycles. Such high performance is attributed to the desirable morphologies, uniform architecture and high surface area. The results manifest that ultrathin Co3O4 nanosheet arrays are promising electrode material for supercapacitor in future application.