Ultrafine (60 nm×10 nm) ZnS:Cu thin film with thickness 200 nm has been prepared by a hydrothermal technique at a relatively low temperature (140 °C). The optical properties of the Cu‐doped ZnS thin film were found to be different from that of the bulk material and normal thin film with large grain size. Compared to the bulk material, the peak position of the blue band shifts to a higher energy (from 445 to 430 nm), and a possible new blue band (peak position at 415 nm) appears as the copper concentration in the ZnS thin film reaches 5×10−3g/g, which is attributed to the equilibrium doping condition and the combination of the Cu+ion with a dangling sulfur bond on the interface of fine grains, respectively. ©1996 American Institute of Physics.