With the advantage of relative low-cost, multi-longitudinal Fabry-Perot laser diodes are widely used in optical communication system. However, the multi-mode characteristics such as multi-mode excitation and mode competition limit the speed or the distance in transmission. Therefore it is necessary to develop a model to understand how the various parameters influence the multi-mode characteristics. In the time domain model [l], the output fields are given by the convolution of input fields and impulse response of the gain filter. The frequency-dependence gain is modeled using a second order Lorentzian digital filter and the phase shift term caused by changes in carrier density is multiplied to the amplitude gain. To find an effective refractive index and a confinement factor that depend on the structure of a laser diode in a section, we used the effective index method . Also the change of the effective refiactive index in each section by the variation of carrier density is considered.