Based　on　Response　Surface　Methodology,　stress　constraints　are　linearized　in　terms　of　direct　variables　through　the　handling　of　internal　force,　and　displacement　constraints　are　approximated　in　first-order　form　dependent　on　reciprocal　variables.　The　cross-sectional　optimization　model　of　truss　with　minimal　weight　is　transformed　into　quasi-unbownded　model　using　the　dual　programming　theory,　which　can　be　solved　by　sequential　quadratic　programming　algorithm.　Its　computational　efficiency　is　compared　to　the　approaches　of　zero-order　and　first-order　approximation　of　stress　constraints　(also　expressed　explicitly　by　RSM)　through　several　numerical　examples,　which　show　the　proposed　method　has　advantages　in　some　aspects.