I-shaped steel plate girders are widely used in various engineering structures, for example, long-span buildings, bridges, gantry cranes, etc. The process of designing a steel plate girder involves determining the size of its web and flanges to minimize weight (or cost) while satisfying design requirements such as ultimate limit state conditions as well as serviceability conditions. This work can be stated as a single-objective optimal design problem, and it can be handled with a variety of algorithms like GA, PSO, DE, etc. However, the practical design sometimes requires balancing two or more conflicting objectives. Minimizing the weight of a steel plate girder while minimizing the depth of its cross-section and minimizing the weight while minimizing its deflection are two examples of multiobjective optimization (MOO) problems. For this kind of problem, there may exist many equally good solutions, forming a Pareto optimal front. In this study, the NSGA-II algorithm is employed to solve MOO problems of I-shaped steel plate girders. There are five sections in this paper. The first section introduces the literature review and the purpose of this study. Section 2 presents the procedure of the steel plate girder design and formulates the MOO problem. Section 3 then gives a brief presentation of the NSGA-II. Section 4 provides an example of how to use the MOO as a decision-aid tool when designing steel plate girders. The last section provides some conclusions.