Power system stability is a problem that has challenged power system engineers for many years. This paper studies transient stability assessment using direct methods by Equal Area Criterion (EAC) and Transient Energy Function (TEF). It introduces a Stability Margin Index (SMI) for the power systems based on kinetic energy. Firstly, the proposed method calculates the critical angles and the critical kinetic energies for all machines in a power system using EAC method. Next, when a fault occurs, by using TEF and the kinetic energy, the system is divided into two bunches: the Critical Machine Group (CMG), and NonCritical Machine Group (NCMG). Then, the stability margin of the system is rapidly judged by using the proposed SMI. By this method, the stability of the system in real-time and at any moment is calculated. Due to the simple calculations, low requirement of data and, nonrequirement post-fault data, the calculation time in comparison to the other ways in which directly potential energy obtained, is much less and appropriate for real-time applications. The proposed method has been applied to IEEE 9 bus test systems and the results show the usefulness and effectiveness of this method.