Colloidal hollow nanoparticles (NPs) of Ni2P have been prepared by a one-pot reaction from a mixture of nickel acetate, oleylamine, trioctylphosphine (TOP), and 1-octadecene. The mechanism to the hollow structure is related to the nanoscale Kirkendall effect. The process contains two important steps. First, oleylamine-stabilized Ni NPs were formed, which can protect them from TOP etching and slow down the inward diffusion of P atoms. Second, the solid-state reaction between the Ni NPs occurred when the TOP concentration and the reaction temperature were correctly adjusted.