A convenient liquid-phase route was proposed to synthesize well-dispersed CuI nanoparticles from Cu+-DMF solution precursor, which was prepared by dissolving bulk Cul in DMF. When water was gradually added, nanoscaled Cul was produced through the decomposition of the complex formed in this solution. A passivation layer of Cu+-DMF adsorbed on the particle surface was suggested to be responsible for the nanoparticle formation.