On the basis of the crystal phase evolution from β-Ni(OH)2 to α-Ni(OH)2, α-Ni(OH)2, β-Ni(OH)2, and their mixed-phase structures were selectively crystallized at different stages during the hydrolysis of Ni(Ac)2 at hydrothermal conditions. We used both XRD characterization and SEM observation to clearly identify such a transformation process. Electrochemical tests indicate that the mixed-phase Ni(OH)2 obtained at 3h possesses the best capacitive performance among all as-obtained samples such as initially formed β-Ni(OH)2 nanosheet aggregates, mixed-phase Ni(OH)2 obtained at other reaction stages, and the ultimately formed α-Ni(OH)2 nanobelts. This phase selective route provides us an effective way to investigate the correlation between crystal phase composition of electrode materials and their capacitive performance.