Nickel sulfide nanoparticles (NPs) are first synthesized by virtue of a unique H2O/CS2 interface under mild hydrothermal treatment. Electrochemical data reveals that the as-synthesized NiS NPs themselves own poor supercapacitive behavior at initial cyclic voltammetry (CV) cycles in 2M KOH solution, while a specific capacitance of 893Fg−1 can be surprisingly obtained at a current density of 5Ag−1 just after continuous 320 CV cycles. X-ray diffraction and Fourier transform infrared techniques demonstrate that what is really responsible for the good electrochemical capacitance in the KOH aqueous solution is the new electrochemically formed Ni(OH)2 phase, rather than NiS NPs themselves. The Ni(OH)2 is slowly formed during the continuous CV cycling process, in which the electrochemically induced phase transformation from NiS to Ni(OH)2 phase takes place. Furthermore, the new Ni(OH)2 phase demonstrates the great ability of delivering large specific capacitance at high rates.