Abstract The introduction of stream division into the separation process that separates a raw material into multicomponent products is effective in reducing the separation cost. However, it is difficult to synthesize this separation process optimally because of information feedback between the separation sequence and introduction of stream division, and the high complexity of the relation between products when the number of products is more than two. The strategy in which two stages—search of the optimal separation sequence and search of the optimal division ratios—are repeated until the objective function is not improved further is adopted in this paper. Manipulation of a diagram which shows graphically the relation between the raw material and the streams outflowing from the separation process, and the functions of separation, division and blending, is effective in searching the optimal division ratios. A useful method for the generation of a suitable diagram (modified material allocation diagram) is developed by considering the species combination of the subproducts decomposed from the product and the outflowing streams. A four-component five-product separation process synthesis problem demonstrates the effectiveness of this method.