Semiconducting transition metal oxide-based nanoarchitectures with chirality are a newly developed research area attracting significant attention because of their wide variety of structures with tunability and their versatile optical/chiroptical properties. In this Article we report the successful synthesis of chiroptically active cobalt oxide/hydroxide nanoparticles passivated by chiral nonthiol ligand proline using a solvothermal approach. Crystalline phases and optical/chiroptical properties of the obtained nanoparticles are dependent on the amount of chiral proline added (or concentration) in the synthesis: In the absence of proline, crystalline cubic-phase Co 3 O 4 nanocubes are obtained, but the use of proline changes their phase to a mixture of Co 3 O 4 , CoO, and β-Co(OH) 2 with amorphous nature, which can be confirmed by X-ray photoelectron and X-ray absorption spectroscopy. Notably, chiroptical responses of the nanoparticles increase with increasing the proline concentration, resulting in the maximum anisotropy factor (g-factor) of about 0.014 in the visible light region. We discuss the origin of the observed chiroptical activity from a viewpoint of chiral distortions of lattices in the nanoparticle. We then anticipate that such chiral metal oxide-based nanomaterials synthesized using proline will be developed as a powerful motif for high-performance asymmetric catalysis and devices in the future.