We generalize the modified double lattice model (mDLM) to investigate the miscibility of polymer blend systems. We re-define model parameters, C-beta and C-gamma, as functions of chain length of polymer and interaction energy. C-beta corrects the degree of asymmetry between two molecules. C-gamma reduces the mathematical defects originated from the truncation of higher order interaction energy terms. We correlate C-beta and C-gamma with Ryu et al.'s numerical data for polymer blends with various chain lengths. In addition, associated interactions between polymer segments are considered based on the secondary lattice concept. In comparison with experimental data, the calculated coexistence curves and critical points by the proposed model show better agreements than those of mDLM. (C) 2004 Elsevier Ltd. All rights reserved.