Nanocrystals-ZnO/polyethersulfone composite film, where ZnO nanocrystals were dispersed in polymer matrix, was prepared by an in-situ method. The crystal size of ZnO was approximately 20nm, showing high dispersion and narrow distribution. The X-ray diffraction (XRD) patterns show that ZnO in the composite film has a hexagonal wurtzite structure. In photoluminescence (PL) measurements, a deep level transition due to defect levels in film is very low, indicating a high quality of the samples.