The problem of transmission network planning is studied thoroughly by applying the mathmatical optimal tecqhniue in this paper. A novel approach is put forward for computer aided transmission network planning, which is quite different from what is based on D.C. load flow equations. A nonlinear transmission planning model is set up by regarding A.C. load flow equations as the system model. A practical model which can be solved more easily, and an united algorithm are obtained by applying the SUMT to the nonlinear model. The results of some practical examples indicate that this method can not only make the planning results more accurate, but also provide the voltage qualities and produce planning plans autometically and seek the plans optimally and fast.