In conventional optimization methods, designers have to set mathematical modeling, such as objective function, constraints and design parameters, strictly and quantitatively. But in actual design process, they do not treat all of these values strictly and some of them are somehow "fuzziness". Recently, many studies have been done on fuzzy mathematical programming. In fuzzy non-linear programming. α-level cut method is considered as the one of the most popular and useful method. But in this method, we cannot see the relationships between fuzziness in constraints and design variables. In this study, we assign fuzzy numbers as design variables, and propose the optimization method using fuzzy number as design variables. We applied the proposed method to a simple truss optimization problem and examined the effectiveness of the method through comparison with the conventional α-level cut method.