It is important to study the influence of close modes on the dynamic characteristics of power systems, and the identification of close modes is a research topic. Based on the analysis of a close modes identification method in the field of mechanics, a modal sensitivity method for identifying them in a power system is proposed. This method is simple in calculation and can effectively identify the close modes. Given modal sensitivity information, the change of oscillation modes can also be determined. Two examples of 10-machine and 24-machine power systems demonstrate the effectiveness of the proposed method. This work is supported by Natural Science Foundation of Hebei Province (No. E2017502075).