In order to determine the most reasonable height of flat vierendeel structure, the stiffness characteristics of flat vierendeel structure are analyzed. Firstly, the conclusion is drawn by analytical method that there is a most reasonable height for any flat vierendeel structure. Secondly, the influence of some main parameters is studied. At last, the calculation formula for the most reasonable height is deduced. Different cases are studied by FEM, and the results confirm the above conclusions and the validity of the formula.