ZnS: Er/Yb nanocrystals were synthesized by the hydrothermal method using thioglycolic acid as a stabilizer. The crystalline phases, morphology, chemical bond states and photoluminescent properties of the nanoparticles were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoemission spectroscopy and fluorescence photometer. The results show that most prepared nanoparticles are spherical in shape with an average size of 5 nm, and have cubic zinc blende crystal structure. The emission spectrum of ZnS:Er/Yb nanocrystals excited at 270 nm displays three main peaks at 470 nm, 530 nm and 580 nm, and the intensity of fluorescence peaks is the strongest for nanocrystals prepared at 120 C. When ZnS:Er/Yb nanocrystals were excited at 980 nm, the emission peaks around 540 nm and 650 nm appeared, associated with the 4F3/2→4I15/2 and 4F9/2→4I15/2 of Er3+ ions transitions.