The Free Electron Maser (FEM) is one of the modern techniques to produced a high power and tunable maser. The FEM does not depend on the explanation of quantum mechanics. The FEM system is composed of many parts. The most important part is the undulator, where the interaction between the microwave and the energetic electrons takes place. This work is an attempt to construct a computational model for the undulator. The simulation technique is used to study the interaction of electrons with the microwave inside a helical undulator. Sixty-four electrons are chosen to cover the phase range 0 – 2 .Three values are chosen for the magnetic field of the undulator, the energy of electron, and the number of period of magnetic field. The result shows the variation of longitudinal velocity of electrons with phase after interaction. The majority of electrons have high velocity. Electrons near the phase n(n = 0,1,2,...) have lower velocities. The longitudinal velocities play the important role in bunch formation, where the velocity modulation is the frame of work of this phenomena. The occurred bunches are along the z-axis of the undulator. We restrict our self to study the bunches inside the undulator only. The variations of the above parameters are quite clear.