The paper presents the development of a computational program that simulates the electromechanical dynamics of an electric power system (EPS). It allows the evaluation of the transient stability and will be integrated into an on-line security assessment system in the northern control area of the Mexican Electric Utility (CFE). Initially, the program was developed for research and academic purposes. The program was written in MATLAB and then in C++ using available public license compiler and numerical libraries. The power system model structure for transient stability analysis is defined by a set of algebraic and differential-ordinary equations (ADOE). This model structure allows the study of several solution algorithms, here an algorithm is used and its particular features are explained. The simulation results of the developed model are validated against the commercial software PSS/E, where good agreement was found.