Nanogranular Co/Ag alloy films with giant magnetoresistance (GMR) were prepared by means of layering discontinuous Co layers and continuous Ag layers alternately. The relationship between the structure and the magnetoresistance (MR) property of such films was investigated. MR ratio varied with the thicknesses of both the Co and the Ag layers and had its peak at around 9.0 Angstrom of Co layer and around 2.5 Angstrom of Ag layer. M-H loops showed that the superparamagnetic particles of the samples with the largest MR ratio in 10 kOe have ferromagnetic coupling between themselves. This means the ferromagnetic coupling helps disordered magnetic moment align and increases the MR ratio under a limited magnetic field.