The definition of marginal line-loss is given firstly.Then the applications to the operation and management of power grid are described in brief

in which the application to economic operation of urban distribution network is stressed.Finally the essential condition of minimizing the line-loss of whole network is got through calculation.By using the conclusions

application software for economic operation of distribution network can be developed to calculate and control the power grid in order to get optimal running state.Applying the proposed software can bring notable economic benefits because of the lower cost

so it is of great significance to be popularized in every power supply bureau.