Abstract: Transmission redundancy and its influence on modern electricity market efficiency are key issues to be considered during planning, investment and expansion of transmission grid. The comprehensive efficiency indicator of modern market is built based on market efficiency and transaction costs theory, then the relationship models of market efficiency, transmission redundancy, transaction costs and excess profits rate are etestablished. In the end, effects of transmission redundancy on modern electricity market efficiency had been analysed in detail through numerical examples. It can be seen from the conclusions that proper transmission redundancy will not reduce market efficiency and optimizing investment of transmission system will improve market efficiency.