ZnCdSe quantum dot (QD) structures have been fabricated under the Stranski-Krastanow (S-K) mode on GaAs substrates by the metal-organic chemical vapor deposition (MOCVD) technique. Prior to the fabrication, the critical thickness of the ZnCdSe/GaAs structure, which plays a crucial role during the formation of the QD, was numerically calculated based on the theory of strain relaxation, and the QD was then prepared in terms of the calculated results. The formation of the QD was confirmed by atomic force microscopy (AFM) and photoluminescence (PL) measurements. By investigating the optical properties of the QD structures with different ZnCdSe thicknesses, the two-dimensional (2D) to 3D transition during the QD formation process was clearly observed, which confirms that the ZnCdSe QD was formed under S-K mode. (C) 2004 Elsevier B.V. All rights reserved.