A facile hydrothermal process with hexadecyltrimethyl ammonium bromide (CTAB) as the soft template is proposed to tune the morphology and size of cobalt hydroxide (Co(OH)2). Monodisperse β-phase Co(OH)2 nanowires with uniform size are obtained by controlling the CTAB content and the reaction time. Due to the uniform well-defined morphology and stable structure, the Co(OH)2 nanowires material exhibits high capacitive performance and long cycle life. The specific capacitance of the Co(OH)2 nanowires electrode is 358 F g−1 at 0.5 A g−1, and even 325 F g−1 at 10 A g−1. The specific capacitance retention is 86.3% after 5000 charge–discharge cycles at 2 A g−1. Moreover, the asymmetric supercapacitor is assembled with Co(OH)2 nanowires and nitrite acid treated activated carbon (NTAC), which shows an energy density of 13.6 Wh kg−1 at the power density of 153 W kg−1 under a high voltage of 1.6 V, and 13.1 Wh kg−1 even at the power density of 1.88 kW kg−1.