Granular (CO, Fe)-M-O (M = Ti, Nd, Al, and Si) thin films showing giant magnetoresistance (GMR) of 4%−10% at room temperature were successfully prepared by the reactive deposition method using an O2 beam. The magnetoresistance (MR) ratios at 19 kOe (MRr (19 kOe)) were found to have roughly a tendency of MRr (M = Si)<MRr (Al)<MRr (Nd)<MRr (Ti), which corresponds to the order of the estimated relative dielectric constants (εr) of amorphous SiO2, Al2O3, Nd2O3, and TiO2. In particular, MRr(M = Ti, 19 kOe) was found to reach a large value of 9.8 (%). Furthermore, MRr (H= ∞) value of these films was found to increase abruptly with the decrease in the average diameter of ferromagnetic particles, and is estimated to reach about 15% at room temperature. The phenomenon can be explained by considering that, as the average diameters of ferromagnetic particles decrease, spin-flips of tunnel electrons are suppressed because the energy for inelastic scattering generated at the time of tunneling between particles with different diameters becomes smaller than the minimum magnon energy.