Abstract: The promotion of renewable uptake level has brought new changes to the power planning. Integrating the system operation risks into the power supply planning and flexibility enhancement is essential to ensure the reliable operation of microgrids. In view of the general low inertia characteristics of a microgrid, this paper proposes a unit commitment efficiency assessment method considering the contingency frequency constraints, the purpose of which is to conduct effective analysis for the power planning considering the system operation risks. First, this paper establishes the concept of unit commitment efficiency, and proposes an indicator termed expected energy not supplied ratio (EENSR). This ratio and the commonly used Conditional Value-at-Risk (CVaR), are taken as the two basic quantitative indicators of the system risks. Secondly, for the given boundary conditions established by the wind power, the solar power, the load demand and the storage, a mixed-integer linear programming model of unit commitment considering the contingency frequency constraints is established to quantify the minimum load loss of a given scenario. On top of that, the classical Monte Carlo simulation is used to summarize the unit commitment energy efficiency. Finally, through case studies conducted on three networked microgrids with distinct features of rich supply, balanced supply-demand and heavy demand respectively, the importance of each unit for the system risk mitigation is analyzed by means of the sensitivity relationship between the risk indicators and the boundary parameters. The case study validates the effectiveness of the proposed method to evaluate the microgrid unit commitment efficiency, and also the instructional value for micro grid planning.