Structures and magnetoresistance (MR) of sputtered Co/Cu multilayers were studied to clarify the influence of the base pressure during film synthesis by preparing the samples under ultrahigh vacuum (UHV) and high vacuum (HV) conditions, with Ar pressures kept constant. X-ray diffraction (XRD) scans revealed that the films were composed of [111]- and [100]-oriented textures in the both cases, and the fraction of the [100] columns was much larger in the HV samples. Secondary ion mass spectroscopy analyses revealed higher oxygen intensities in the HV samples especially in the early stages of film synthesis. The XRD indicated that periodic structures were formed in all samples, and suggested that oxygen atoms were localized at Co-Cu interfaces and grain boundaries in the HV samples. Transmission electron microscopy observations disclosed that there was no significant difference in Co-Cu interface roughness between UHV and HV samples. MR measurements showed that the MR ratio was larger in the UHV samples. The difference in the MR ratio was mainly due to the difference in saturation field resistivity and in orientation of textures, but about 7% difference remained even after subtracting these factors. This was presumably ascribed to the more ferromagnetic character of the HV samples, possibly brought about by the impurity oxygen.