The main aim of this study is to investigate the effect of annealing on magnetotransport and on structural properties of Co/Cu multilayer with Ta buffer layer, prepared by dc magnetron sputtering. The heat treatment of the sample at 250 °C up to 20 h induced a prominent change in giant magnetoresistance, which correlates with the corresponding changes in remanent magnetization. In order to investigate the origin of the changes in the magnetotransport properties, the microstructure of the samples has been studied by x-ray diffraction. The structural modifications were elucidated by comparing x-ray spectra calculated using a nonideal step model with measured spectra and they are related to the changes in magnetotransport properties. A correlation between the changes in remanence and interlayer exchange coupling has been established using a modified Stoner-Wohlfarth [Philos. Trans. R. Soc. London, Ser. A 240, 599 (1948)] model.