The influence of impurity gas in the sputtering atmosphere on the microstructure and the GMR was investigated on Co/Cu multilayers. The MR ratio increased from 12 % to 27 %, when the chamber pressure, Pb before introducing Ar was changed from 5x10(-8) Torr to 2x10(-5) Torr by leaking air. From structural analysis, the multilayer fabricated with P-b=2x10(-5) Torr has a small grain diameter and a relatively fiat stacking structure. Co layers in the multilayer fabricated under highly purified atmosphere and excessively contaminated atmosphere, behaved as particulste in lower temperature.