Nearly monodispersed CuIn0.7Ga0.3Se2 (CIGS) nanocrystals were successfully prepared using the thermal decomposition of copper, indium (In), gallium (Ga), and selenium-oleylamine (OLA) complexes via the heating-up process. The phase formation sequence was CuSe -> CuInSe2 -> CIGS. The CIGS nanocrystals synthesized using the heating-up process exhibited a single chalcopyrite phase and its chemical composition was close to stoichiometric. However, phase separation of the In-rich and Ga-rich phases was found using the hot-injection method due to the difference in reactivity between In-OLA and Ga-OLA complexes.