The formation process of CdSe self-assembled quantum dots (SAQDs) below the critical thickness was observed by atomic force microscopy (AFM) for the first time. Two monolayers (MLs) of CdSe coverage were grown directly on GaAs (1 0 0) surfaces by metalorganic chemical vapor deposition (MOCVD). AFM images were taken constantly of the same area of 1 mum(2) within several hours after the growth. It revealed that the formation of CdSe SAQDs under critical thickness was due to the effect of surface diffusion and strain release. Our results make it possible to directly observe the process of release strain and to obtain the actual information on the formation process of self-assembled quantum dots.