Ultrafine cobalt-nickel bimetallic phosphides were prepared by mild hydrothermal method and characterized by techniques, such as XRD, EDS, and TEM. The results showed that the as-prepared products were well crystallized and particle sizes ranged from 10 to 20 nm. With the increasing amount of Ni in the suspensions, the crystallinity of obtained products was improved gradually and the crystallite sizes increased accordingly.