Nanometer β-Ni(OH)(2), showed by XRD, was prepared by our supersonic coordination-precipitation method, with an average grain size of about 50 nm by TEM. Proton diffusion coefficient of nanometer Ni(OH)(2) and spherical Ni(OH)(2) were 1.93 x 10(-11), and 5.50 x 10(-13) cm(2)/s, respectively, with combination of chronocoulometry and cyclic voltammetry. Charge-discharge test of simulated batteries at 0.2° C showed that addition of 8 mass% 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 50 mAh/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). © 2004 Elsevier Ltd. All rights reserved.