To study the connections of main columns of lattice steel tube transmission towers, a flange design method which ensures the strength of both the flange itself and the bolt is proposed, based on the results of experiments and FEM analyses. A stress calculation method is developed using the axisymmetric finite element method, in which the Young's moduli of ring elements of bolt and bolt hole are reduced according to the ratio of total cross-sectional area to ring element area. The applicability of the calculation method was confirmed by stress measurements of both flange and bolt in full-scale models, in which the initial induced load of the bolt was varied. Using the calculation method, the contour of flange which provides sufficient strength for use in large power-transmission towers can be determined.