A Monte Carlo simulation for the segregation behavior of symmetric diblock copolymer at the interface between immiscible homopolymers was performed on a simplified lattice model. The simulation results show that the segregation behavior of block copolymer is significantly dependent on the molecular parameters, such as the chain length N-c of block copolymer and the interaction energy epsilon(AB) between A and B segments. In a weakly repulsive system with epsilon(AB) = 0.1, the block copolymers are more strongly segregated at the interface with increasing the chain length when the block length is shorter than and comparable to the homopolymer length. However, when the block length becomes longer than the homopolymer length, the concentration of copolymer segments at the interface rather decreases and its distribution becomes broadened. In a strongly repulsive system with epsilon(AB) = 0.5, the distribution of copolymer segments at the interface becomes broader as the chain length of block copolymer increases. This behavior is closely related to the stretching and orientation of block copolymer chains at the interface and to the extent of penetration of homopolymers into the copolymer layer. These simulation results are in excellent agreement with the numerical predictions by the self-consistent mean field theory.