Abstract: Using the bond-fluctuation-model and vacancy-diffusion-algorithm,the microphase structure, segment profile and structure factor of molten symmetrical AB diblock copolymer confined between two impermeable plate surfaces are simulated by Monte Carlo method based on simple cubic lattice.It is shown that microphase is lamellar and lamellar orientation is determined by the characteristics of plates.The lamella between two neutral plates is perpendicular to plates.That between two symmetrical preferential plates or asymmetrical preferential plates is parallel to plates.If one of plates is attractive and the other is neutral, the microdomain is perpendicular lamellar structure near neutral plate and parallel lamellar structure near attractive plate.The structure factor shows that microphase structure of symmetrical system is mainly decided by the symmetry of external field.