Magnetic and magnetoresistance properties as functions of Co composition and annealing temperatures were investigated in the Cu_(1-x)Co_x (x=0.2∼0.4) alloy films deposited by thermal co-evaporation. The MR ratio of 12.5% at room temperature which was the largest among the reported values was obtained in the 25 at.%Co-Cu granular alloy film aged at 500℃ for 140 minutes. At higher measuring temperature and limited external field, the maximum MR ratio occurs for larger Co size. In the low Co concentration($lt;25at.%) alloy films, they behaved mainly as paramagnetic in the as-deposited state at room temperature and above that composition, it showed the ferromagnetic character. The enhanced MR ratio up to annealing temperature of 500℃ was due mainly to the reduction of the Cu matrix resistivity, which resulted from the Co precipitation and Cu matrix grain growth. The MR ratio and the magnetization decreased by raising the annealing temperature from 500℃ to higher temperature. These were thought to be due to re-solution of precipitated Co back into the Cu matrix at higher temperature. The Co content of the diffused interface layer decreased and a result of that, the interface spin-dependent scattering power decreased.