This paper aims to present the design of a computer code Transient Stability Code (TRANS_STAB_CODE) for the Transient Stability studies of electrical power systems under the influence of harmonics, using the GUI in MATLAB. The program was run for the loads are linear and nonlinear (Static Var Compensator (SVC), Thyristor controlled Reactor (TCR), and Unified Power Flow Controller ((UPFC)), After describing the program was run for five networks IEEE: 6 Nodes 3 Machines, 09 Nodes 03 Machines, 10 Nodes 02 Machines, 13 Nodes 04 Machines and 14 Nodes 05 Machines. The Transient Stability studies by the methods of: Runge Kutta, Euler, and Heun. The results were almost consistent and show the influence of higher harmonics on Transient Stability in electrical networks.