Abstract: The model of structural fuzzy optimum design of offshore jacket platforms is established considering the fuzziness of constrained boundary condition. Design variables, objective function and constrained conditions in the fuzzy optimum model are considered with fuzzy method. According to the characteristics of offshore jacket platforms, the tolerance coefficient of the constrained boundary condition is determined with fuzzy optimization method. The optimum level cutset λ^*, which is the intersection of the fuzzy constraint set and fuzzy goal set, is determined with bound search method, and the fuzzy optimum solutions of fuzzy optimization problem are obtained. The Chengbei No. 11 offshore platform, which lies in the Shengli Oil Field, is designed with the present fuzzy optimum model, and the results are compared with those of the deterministic optimum design. The tendencies of design variables in the two methods are discussed. It is found that the value of objective function in the fuzzy Optimum design is lower obviously than that of the deterministic optimum. Therefore, the optimum design for jacket platforms considering fuzzy factors is necessary and feasible.