Quantum dots with a ZnS/CdSe/ZnS(core/well/shell) composition are synthesized using a one-pot method, and their structural and optical properties are investigated. Compared to the conventional CdSe/ZnS core shell structure (CS), ZnS/CdSe/ZnS quantum well shell structure (QWS) demonstrate remarkable optical properties, probably because of multiple exciton generation and long-range Wannier excitons, although they show similar crystal structure, crystallite size and energy band gap. The one-pot synthesis is a useful method for growing not only CS but also QWS.