The profiles of junction current and voltage and carrier-concentration in an injection laser with filament were calculated on the basis of a simple model. Numerical results indicate how the various parameters such as bulk resistivity and carrier diffusion length in the active region affect the profiles. Particularly, the filament width is strongly dependent on the bulk resistivity. The model connects the stimulated radiation characteristics of the device with the measurable quantities of the external circuit. It shows that there is obvious difference between average density of operating current and current density in the filament rigion. It shows also that nonuniformity of material will play an important role in locating the filament.