A one-dimensional model of free electron laser (FEL) was presented through a computer program in Matlab 6.1 language. The FEL type was chosen to be oscillator with helical undulator. The input electron beam is uniform with initial phases uniformly distributed along the interval (0-2?). The work deals with the linear mode of operation. The effects of undulator parameters and space charge parameters on the output power gain, the effects of number of round-trips in the resonator on the intracavity power and power loss ratios, the threshold condition and the operation below and above threshold were studied. It was found that the output power gain increases as the undulator length, the electron beam density and the magnetic field strength increase. It was also found that the increase in space charge parameters causes a decreasing in the output power gain of the FEL. The intracavity power and power loss ratios are increased as the number of round-trips in the resonator increased. Finally the gain of micro undulator was studied by considering new values of undulator parameters.