Ab initio investigation of the structural and electronic properties of a real size Mo27S54 cluster has been performed to develop a fundamental understanding of the active sites of MoS2 catalysts in the hydrotreatment process. The cluster is a stoichiometric and regular hexagonal one with ((1) over bar 0 1 0) plane (S edge) and (3 0 (3) over bar 0) plane (Mo edge) only, and does not need any saturating H atom and charge compensation. By using density functional theory (DFT) method, a full geometry optimization of the cluster has been carried out. The structure of the cluster is more relaxed towards the edges, and two types of the Mo-Mo distances, which are shorter or longer than that in the MoS2 crystal, are observed at the periphery. These results agree well with the EXAFS data of dispersed unsupported MoS2 particle. The electronic properties of the atoms at various sites (i.e. comer, edge, outer, and inner positions) have been distinguished clearly by means of charge distribution and molecular orbital (MO) calculations. The comer Mo atom is expected to be the most active site for the hydrotreatment reactions. (C) 2002 Elsevier Science B.V. All rights reserved.