This research proposes a systematic approach to identify critical assets that support the efficiency of electricity markets. NERC standards CIP provides guidance on the identification and protection of critical cyber assets that support the reliability of power systems. Those assets don't include properties that are critical to the efficiency of electricity markets. However, attacking such properties may cost electricity market customers millions of dollars. This systematic approach fills the gap, and identifies the assets that have significant impacts on the market efficiency. The methodology is to analyze congested flowgates in an electricity market. It starts from a full list of the congested flowgates, and rank the congested flowgates with Estimated Potential Benefit (EPB) calculation. Top congested flowgates are then obtained. Apply correlation calculation to the top congested flowgates. A final list of critical congested flowgates is generates as a result. The simulation is performed on an ISO market. It identifies a short list of critical congested flowgates in the ISO market. Attacking one of those critical congested flowgates may cost the ISO market averagely thousands of dollars per hour. Future study is to develop security mechanisms for the critical congested flowgates. It includes both cyber security measures and physical security measures.