The One-dimension theory is applied to study the performance of free electron laser amplifier when the electron beam have a uniform density distribution in the direction perpendicular to the helical undulator axis with initial phases uniformly distributed along the interval (0-2?). The matlab 6.1 program is used to built the computer programs that used in this work. In the present work, the motion of electron are considered inside the helical undulator. The equations of motion for electron inside the undulator, show that three effecting parameters, they are: 1- Undulator magnetic field 2- Undulator period . 3- Lorentz factor The increase in undulator magnetic field leads to increase in the radius of electron trajectory, and the magnitude of transverse electron velocity and at same time leads to decreasing the magnitude of the longitudinal electron velocity. The above effects of undulator magnetic field be reapted in the case of undulator period. Owing to the differences in the equations that used in this case, it is found differences in waves behavior of electron. At last, the increase in Lorentz factor leads to decreasing in the radius of electron trajectory and magnitude of transverse electron velocity and also causes an increasing in magnitude of the longitudinal electron velocity. The effects above discussed as a result of the law of the conservation of energy inside the undulator. The motion of electron is also considered under the effect of the presence of electric field that may be as an electrical component of electromagnetic wave (laser). As comparing with first case (when there is no electric field), there are increasing in the magnitude of longitudinal electron velocity and a variation in electron energy. Within this case, the effect of the phase of electron, undulator magnetic field and undulator period are studied .