We used density functional theory (DFT) to investigate the formation of symmetric and asymmetric thin film of diblock copolymer melts by tuning the size of the slit confinement. In this work, the DFT contains a modified fundamental measure theory for the excluded volume effect and the first-order thermodynamic perturbation theory for the chain connectivity as well as the mean-field approximation for van der Waals attraction. For the symmetric A(8)B(8) linear copolymers, it is observed that with the increase of the width of the slit, morphologies of copolymer in the slits undergo an evolution of "nonlayered structure -> ABA -> ABAB -> BABAB -> disordered structure'', while the morphologies of asymmetric copolymer with the increase of the width of the slit exhibit a process of "ABA -> ABAB -> ABABA -> ABABAB lamellar structure'' in all the cases studied. It suggests that the ratio of two blocks of a copolymer plays an important role on the structure of copolymer film. By adjusting the ratio of two blocks, some copolymer films with novel morphologies, including asymmetric ABAB lamellar structure, can be tailored. Furthermore, it is found that the bonding orientation distribution introduced into the DFT can act as a criterion to identify the disordered and ordered states of copolymers. (C) 2009 Elsevier B.V. All rights reserved.