In this work it was studied the influence of large quantities of iron in the structural and magnetic properties of copper oxide nanoparticles, which were produced by the co-precipitation method. The samples studied were those of the (Cu1 − xFex)O system, with x = 0.20, 0.25, and 0.30. The concentrations of the precipitating agent were 7 and 8 M, and with variable calcination temperature. Based on the Rietveld refinement of the X-ray diffraction patterns, all the samples were found to have only the CuO monoclinic structure with a C2/c space group. In addition, it was found that the mean crystallite size varies between 39 and 59 nm, and that the substitution of Cu by Fe atoms occurs along the a-axis. By SEM measurements, it was detected that the particles are present as agglomerates. Previous work reported for the sample with x = 0.15, and prepared by co-precipitation [1], proved that the obtained particles are of nanometer size. The zero field cooling (ZFC) and field cooling (FC) curves, obtained by vibrating sample magnetometer (VSM), showed a peak at 97 K, which was also detected for a sample with x = 0.15, which corresponds to a reentrant spin-glass transition [1].