Abstract: To solve the efficiency issue on modern electricity market, efficient market hypothesis theory of finance and transaction cost theory of economics are introduced in research on modern electricity market, which break traditional analyzing framework and buid a novel one of modern electricity market efficiency and power transaction cost. Then stochastic model of modern electricity market efficiency based on efficient market hypothesis is proposed, and three efficiency identifying indices, such as pricing reaction extent to information, excess profit rate and the proportion of transaction costs, are also given.