In this paper, we reported the preparation of non-stoichiometric copper sulfide Cu7S4 nanocrystalline by ultrasonic irradiation in ethylenediamine solvent system mixed with a complexing agent of 1-decanthiol, using sulfur powder and copper nitrate as reactants under normal laboratory conditions. X-ray diffraction (XRD) pattern indicates that the products are nanocrystallites in monoclinic structure. Scanning electron microscope (SEM) images display that the product consists of nanocrystallites of about 20 nm which aggregated in the form of polydispersive nanoclusters with sizes in the range of 150-300 nm. Energy-dispersive X-ray analysis (EDAX) shows that the products are absolutely pure and atomic percentage ratio of Cu to S is about 60:40. UV vis absorption spectroscopy of the as-prepared particles reveal an energy band gap of about 2.54 eV; compared to 2 eV corresponding to its bulk value; a blue shift of about 0.54 eV has been observed, which is understood as quantum size effect due to three-dimensional confinement of electrons and holes in a small volume. (C) 2007 Elsevier B.V. All rights reserved.