Chalcopyrite-type CuInSe₂nanoparticles are successfully prepared by using In₂Se₃nanoparticles as a precursor reacted with copper chloride (CuCl) solution via a phase transformation process in low temperature. The reaction time is a key parameter. After the reaction time increasing from 0.5 h to 8 h, In₂Se₃and CuCl react with each other gradually via phase transformation into CuInSe₂ without any intermediate phase. The crystalline structure and morphology of the CuInSe₂nanoparticle₂are characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The diameter of CuInSe₂ nanoparticles with good dispersibility ranges from 10 nm to 20 nm. The band gap of the CuInSe₂nanoparticles is 1.04 eV calculated from the ultraviolet-visible (UV-VIS) spectrum.