The controlled synthesis of transition metal phosphides has been pursued to obtain excellent performances in application. Herein, we report a simple and effective method to synthesize nickel phosphide nanoparticles with target phases. Pure-phase nickel phosphide nanoparticles were obtained in different crystalline states (Ni2P and Ni12P5), and the crystalline phase of nickel phosphide could be controlled by varying the reaction conditions such as the temperature and duration of thermal treatment or the ratio between Ni and P. In addition, the nickel phosphide particles after thermal treatment maintained their sizes without serious agglomeration. In the hydrogenation of nitrobenzene, the phosphides with pure-phase (Ni2P or Ni12P5) and high crystallinity showed high catalytic activities. This proves that the crystalline phase of nickel phosphide plays an important role in the catalytic activity.