In mixed model assembly line, products are assembled sequentially that have different combination of options which customer specifies. In JIT environment, production smoothing becomes an important issue to sub-line which supply the necessary parts to each work station of the assembly line. Sequence of the mixed products put into the assembly line affects substantially the number of line stoppage and the inventory level of each part. To find a sequence which minimizes the costs associated with the line stoppage and the inventory level, a mathematical model is proposed. Recognizing the limit of the Branch and Bound technique in large size problem, a heuristic solution is proposed based on pairwise interchange method. The validity of the heuristic is by solving example problem : The results are compared with the Branch and Bound technique in small size problem and the best of many random solution in case of large size problem.