A PMU-based algorithm of synchronous generator stability prediction is proposed and tested in the paper. The algorithm is based on the application of the equal area criterion for the following spaces: active power - load angle, active power - time. The deceleration area is formed using the least squares curve fitting by the second order polynomial. The algorithm was tested on the single synchronous machine power system. A number of transients was simulated with the power angle curve polynomial accuracy of 50 ms. The results have shown high adaptability and efficiency of the proposed algorithm. It can be used in adaptive protection and control devices to form control actions aimed at ensuring the stability of a power system operation.