The intergrain magnetoresistance (IMR) was observed in a number of different polycrystalline half-metallic ferromagnets at temperatures below the Curie temperarure. By taking into account the basic physics of ferromagnetic semiconductors, we generalize the spin-polarized intergrain tunneling model to theoretically study the IMR effect in the polycrystalline ferromagnetic semiconductors. It was shown that the IMR ratio is strongly dependent on both the itinerant-carrier density and the magnetic-ion density in the ferromagnetic semiconductor grains. The larger IMR may be achieved by optimally adjusting the material parameters.