Tunnel magnetoresistance (TMR) effect in ferromagnet/granular/ferromagnet (FM/GR/FM) tunnel junctions was studied, where GR = Al-O/Co/Al-O or Al-O/Co/Al-O/Co/Al-O. The magnetoresistance (MR) ratio and the resistance of the latter junction increased with decreasing bias voltage at low temperatures. These increases should be due to the cotunneling effect in the Coulomb blockade regime. In contrast, the MR ratio of the former junction exhibited a maximum of 14% near 40mV and decreased to 10% at 1 mV. The increase of resistance in the former junction at a lower voltage was smaller than that in the latter junction. The difference was discussed using a simple model.