The ZnS: Er nanocrystals were synthesized by the hydrothermal method with thioacetamide as the sulfur sources. The phase, morphology, components and luminescent properties of the nanocrystals were characterized by Xray diffraction (XRD), Xray photoelectron spectroscopy(XPS), transmission electron microscopy (TEM) and fluorescence spectrophotometer. The results show that ZnS∶Er nanocrystals have cubic zinc blende crystal structure with an average size of 5 nm. Zn、S、C、O and Er elements are found in the ZnS∶Er nanocrystals from XPS spectra. ZnS∶Er nanocrystals have two emissions peaks at 469 nm and 583 nm, the two emissions peaks become more intensive and shift towards blue side as the value of the pH increases, which is the strongest performance at pH=12. The intensity of the peak at 469 nm enhances as the Erdoped content enhances initially and weakens afterwards, but the intensity of the peak at 583 nm always weakens.