Nanometer β-Ni(OH)2, showed by XRD, was prepared by our supersonic coordination–precipitation method, with an average grain size of about 50nm by TEM. Proton diffusion coefficient of nanometer Ni(OH)2 and spherical Ni(OH)2 were 1.93×10−11, and 5.50×10−13 cm2/s, respectively, with combination of chronocoulometry and cyclic voltammetry. Charge–discharge test of simulated batteries at 0.2°C showed that addition of 8mass% of our prepared nanometer Ni(OH)2 in nickel hydroxide electrodes led to increases in cathode discharge specific capacity (CDSC) by nearly 10% and the chargeability of the electrode by about 50mAh/g, and a decrease in polarization. Cycle life test of AA-type MH-Ni batteries discovered that effect of nanometer Ni(OH)2 in increasing CDSC was more apparent for the first 100 cycles and not much difference was found after 350 cycles. XAS demonstrated a higher oxidation state of Ni in fully charged nanometer Ni(OH)2 composite electrode (Nano-E) and a lower one in discharged Nano-E, compared with micrometer Ni(OH)2 spherical electrodes (Micro-E). A larger structure distortion was found in Nano-E, offering more vacancies for proton diffusion. Thus conversion between Ni2+ and Ni3+ was promoted during the charge–discharge process, which was assumed to be one explanation of increasing CDSC with the addition of nanometer Ni(OH)2.