The purpose of this paper is to describe a Network Programming Model for least-cost investment in electric power generation and transmission system and illustrate it by a case study involving application of this model to northern Region of India for planning the electric power generation and transmission system network to meet projected peak demand at the end of Fifth Plan Period. The model described in this paper is essentially an economic model based on the power system network in which electric power flows from generating nodes to load centres through existing or proposed transmission lines and costs of generation of power and transmission including power and energy losses are accounted for. The model with the aid of a network computer programme obtains a least-cost flow pattern in the network which determines the optimal investments in generating plants and transmission network. The optimal solution obtained should be tested with load flow studies to determine system reliability under various contingencies as is done in case of any other modelling approach.