The objective of the paper is to present a -t stability index that can be used for the security assessment of electric power systems. This index provides an evaluation of the system securitylevel by considering t he values of the accelerations and kinetic energies of the generators, after the occurrence of a large disturbance. The derivation of the index is based on the pattern recognition principles and the fuzzy sets theory. The results that are o btained f rom the application of the i ndex are illustrated by the stability analysis of two power systems. The first is the CIGRE test system, while the second is the Hellenic Interconnected System.