We extended and simplified the modified double-lattice model for polymer solution systems to binary polymer blend systems. The model has three model parameters, C-beta and C-7 in a primary lattice and C-alpha in a secondary lattice. Those are not adjustable parameters but universal constants. In comparison with Muller's Monte-Carlo simulation data for symmetric polymer blend (r(1) = 32 and r(2) = 32), C-7 is determined and C-beta is negligible for symmetric polymer blend systems. Also, C-alpha is obtained from comparing with Panagiotopolous, Monte-Carlo simulation data. In addition, oriented interactions between polymer segments are considered based on the secondary lattice concept. The proposed model describes very well phase behaviors of ordinary and oriented polymer blend systems. (C) 2003 Elsevier Science Ltd. All rights reserved.