Alkaline rechargeable batteries, such as Ni/Cd batteries and Ni/MH batteries have been widely used as power sources, however, their further applications are limited due to contamination of Cd in Ni/Cd batteries and lower discharge capacities of Ni/MH batteries. Some metal borides, such as Co-B, Ni-B, Fe-B, V-B and Ti-B, have been known to own very high discharge capacity in alkaline aqueous solution, in which Co--B alloy exhibits the highest reversible discharge capacity and the best cyclic stability. However, the reversible capacity of Co-B alloy prepared by arc melting process is usually less than 250 mA?h/g, which is only one fourth of the theoretical capacity of the alloy electrode (908 mA?h/g). In the present study, chemical reduction method was used to prepare Co-B alloy to further enhance the electrochemical capacity of the alloy. A series of ultrafine powders of amorphous Co-B alloys, Co_0.68B_0.32, Co_0.55B_0.45 and Co_0.50B_0.50, were prepared by reducing CoSO₄ with solution of NaBH₄. Electrochemical measurements indicate that the prepared alloys exhibit excellent electrochemical properties. At a high current density of 300 mA/g, the initial discharge capacities of these alloys are 510.6, 666.4 and 667.2 mA$?h/g, respectively, their discharge capacities still keep 331.6, 379.5 and 390.5 mA?h/g after 60 cyc, respectively. Even at a discharge current density as high as 1200 mA/g, the three alloys still deliver reversible capacities of 336.2, 373.4 and 390.1 mA?h/g, respectively. In the Co-B alloy electrodes, the boron atoms have two functions. First, boron can be oxidized to BO₃^3-, thereby partly contributes to the discharge capacity. Second, most importantly, the gradual dissolution of boron into the electrolyte (in the form of BO₃^3-) during the charging/discharging creates a new surface in the electrodes, which can effectively increase the surface area of the active material in contact with the electrolyte. The alloy with higher boron content thereby can produce a larger reaction surface area by boron dissolution than the alloy prepared with lower B content. So the higher B content in the Co-B alloys can be helpful for improving their electrochemical properties.