Fast transient instability detection tools are required in stressed power system to avoid widespread power outages. In this paper, an instability detection method is developed for real-time application in the wide area monitoring system. The proposed method uses frequency measurements that are readily available from phasor measurement units. In order to subside the impact of rapid changes in the measurements on the instability detection, a frequency trend vector is considered. A stability index is introduced to evaluate the power system stability. Unlike stability prediction and forecasting, this paper focuses on present stability status of the system. The methodology proposed is demonstrated by using a 39 bus system. © 2016 IEEE.