In the restructured power industry environment, the economic impact assessment on transmission expansion from a societal perspective is indispensable. This paper proposes a method on this subject. Firstly, we introduce the classical economic assessment indices, and then present several basic concepts on power industry economics. Based on these, the paper proposes an integrated economic impact index on transmission expansion. The index takes the equity aspect as well as the efficiency aspect into account. The concepts of the classical utilitarianism social welfare function and the Lorentz curve and the corresponding Gini coefficient are introduced to measure the efficiency impact and the equity impact of a transmission expansion, respectively. The index can be used to guide the search or the selection of an optimal transmission expansion scheme from the societal perspective. Next, we use a six-node system case to illustrate the application of the index proposed. Then, we present a conclusive statement to end the paper.