The composition and thickness dependence and the ferromagnetic under- and overlayer effect on the magnetoresistance ratio and saturation field of the Co-Ag nano-granular films were investigated. The maximum magnetoresistance (23% at R.T.) in the as-deposited state was obtained in the $3000{AA}$ $Co_{30} Ag_{70}$ bare alloy film. As the thickness of the alloy films decreased below $500{AA}$, the MR ratio decreased because of the resistivity increase and the non-uniform film formation. We showed that the ferromagnetic over- and underlayer could reduce the saturation field of the nano-granular films via exchange coupling effect. The magnetoresistance and the saturation field of the $100{AA}$ alloy film were 3.65 % and 2.85 kOe respectively and those of the under- and overlayered alloy films with $200{AA}$ Fe were 3.3 % and 1.23 kOe respectively.