In this paper, the impact of modular product designs on the performance of manufacturing systems is studied. The performance of product designs is measured by the makespan of the corresponding aggregate schedule of the manufacturing system. Design rules for the improvement of performance of a manufacturing system are developed. Based on the design rules developed, the selection problem of modular designs is formulated as an integer programming problem. The problem can be solved by an existing heuristic algorithm. Examples are provided to illustrate the issues discussed in this paper.