Much of the decision-making in the real world takes place in an environment in which the goals, the constraints and the consequences of possible actions are not known precisely. To deal quantitatively with imprecision, the tools of fuzzy set theory can be used. This paper deals with the description and optimization of mechanical systems containing fuzzy information. The fuzzy constraints define a fuzzy feasible domain in the design space and hence the fuzzy optimum solution will be defined by a fuzzy set of points. In this work, two methods are presented for solving a fuzzy optimization problem using ordinary optimization techniques. The optimum design of a four-bar function generating mechanism with fuzzy objective function and fuzzy constraint set is considered to illustrate the procedures.