The ternary I-III-VI₂ semiconductor of CuInSe₂ nanoparticles with controllable size was synthesized via a simple solvothermal method by the reaction of elemental selenium powder and CuCl as well as InCl₃ directly in the presence of anhydrous ethylenediamine as solvent. X-ray diffraction patterns and scanning electron microscopy characterization confirmed that CuInSe₂ nanoparticles with high purity were obtained at different temperatures by varying solvothermal time, and the optimal temperature for preparing CuInSe₂ nanoparticles was found to be between 180 and 220 °C. Indium selenide was detected as the intermediate state at the initial stage during the formation of pure ternary compound, and the formation of copper-related binary phase was completely deterred in that the more stable complex [Cu(C₂H₈N₂)₂]⁺ was produced by the strong N-chelation of ethylenediamine with Cu⁺. These CuInSe₂ nanoparticles possess a band gap of 1.05 eV calculated from UV–vis spectrum, and maybe can be applicable to the solar cell devices.