We have investigated the temperature_dependent photoluminescence of two types of quantum islands in ultrathin CdSe/ZnSe layers. At low temperatures, excitons in these two types of quantum islands can transfer by tunneling. The hopping process of the localized excitons gradually becomes the dominant mechanism with increasing temperature. We found that the small islands are nano_scale clusters with quasi_zero_dimensional properties, which are different from those of the large islands. Experiments indicated that these two types of islands are optically correlated although their excitonic properties are different.