Different-sized CdSe nanocrystals were synthesized in aqueous solution and were transferred from water into organic solvent using a surfactant. Nanocrystals/polymer hybrid light-emitting diodes were fabricated using the blends of different-sized CdSe and poly(N-vinylcarbazole) (PVK) as emissive layers. By measuring the electroluminescence (EL) properties of the hybrid devices, we found that the excitons were generated in CdSe nanocrystal layers by direct recombination of holes and electrons into CdSe nanocrystals or by the Forster energy transfer of excitons from PVK to CdSe nanocrystals. The photoluminescence spectra of the CdSe:PVK composite films demonstrated that energy transfer from PVK to CdSe was more efficient for the composite film with smaller-sized CdSe nanocrystals. The EL spectra of the hybrid devices with the smallest nanocrystals displayed only one peak, and the origin of the EL emission was clarified by studying the EL spectra under different applied voltages and different mass ratios of CdSe to PVK. (C) 2008 The Electrochemical Society.