Cu-doped ZnS nanocrystals were prepared in an inverse microemulsion at room temperature as well as under a hydrothermal condition. X-ray diffraction analysis showed that the diameter of the Cu-doped ZnS nanocrystals particles was about 9 nm. These particles showed a strong photoluminescence intensity and a broad emission band from 490 to 530 nm. The half-width of emission was about 60 nm. Cu-doped ZnS nanocrystals/polymethylmethacrylate composite as a light-emitting layer was used to fabricate a single layer structure electroluminescent device which had low turn on voltage (less than 5 V). The green light of electroluminescence was observed at room temperature. The electroluminescence and photoluminescence spectra were nearly identical at room temperature. ©1998 American Institute of Physics.