This paper deals with a design optimization system for a fast reactor. The system consists of a nonlinear programming code and the usual design codes. If proper design criteria and an initial estimate of a design point are provided, the optimum design point, subject to those criteria, can be automatically obtained by the system. The system was successfully applied to the optimization and sensitivity analysis of a typical large fast reactor. The results show both the features of the optimized reactor and the effects of variations in the main design criteria on the design.