An optimum cross section of a precast modular bridge was drew by using the combinational optimization method. The constraints for the optimization procedure including the width and the thickness of the bottom flange, and the maximum area of the cross section, etc. were considered, and weight of the girder was minimized to draw an optimum cross section. Furthermore, the segmentation of the girder in its longitudinal direction was studied for satisfying the requirement of traffic law about overloading.