In electricity market, the power transactions needs to be executed under strict physical and other constraints, such as transmission congestion which may lead to the decrease of market efficiency. In the paper, under considering transmission congestion, it establishes the model that reflects market transaction, including generator output and sale price. Then it gives index system which is used to evaluate market efficiency under congestion. Finally, it utilizes IEEE 30-bus test system to illustrate the application of proposed method. The numerical results show that the transmission congestion may reduce social welfare and bid up sale price. And based on optimal power flow model, it also provides an efficient way of relieving congested line.