In recent production environment, many factories introduce mixed production to produce multi-types of products. Ordinarily, multiple work elements are assigned to appropriate workers or machines for process design to aim at high productivity in mixed production, and production ratios of the products are considered for the design. However, this process design is inappropriate to construct effective production line when the production ratios of products are continuously changed. In addition, when quantities of products are changed by lack of supplied parts and a change of prediction of production, this method is inappropriate. In order to treat these issues, levelization is introduced into mixed production line. However, the method for levelization in the line is not discussed sufficiently. In this study, we deal with of process design for mixed production line for levelization and minimization of total in-process inventory in the line. Mathematical model is constructed for process design to promote levelization and to reduce in-process inventory. Numerical experiments are performed to evaluate the productivity of the line designed by resolving mathematical model. The performance of the production line is evaluated by using event-driven simulation. The simulation result shows that the production line generates high productivity under the conditions of different production ratios of products and reduces in-process inventory.