We present a mesoscopic off-lattice model for the simulation of diblock copolymer melts by Monte Carlo techniques. A single copolymer molecule is modeled as a discrete Edwards chain consisting of two blocks with vertices of type A and B, respectively. The volume interaction is formulated in terms of coarse-grained densities of the A and B vertices. The model allows the study of equilibrium conformational properties of diblock copolymer microphases for sufficiently large system sizes.