Magnetism of a very thin antiferromagnetic (AFM) surface CuO has been investigated with partially oxidized nanocomposites of Cu–CuCl, ~200 nm. The samples are characterized by x-ray diffraction, x-ray photoelectron spectroscopy, x-ray-excited Auger electron spectroscopy, transmission electron microscopy and magnetic measurements. The characterizations indicate that the composites have a core–shell structure. Before oxidation, it is (Cu)_core/(CuCl)_shell, and after oxidation it is (Cu)_core/(Cu₂O+CuCl+minute CuO)_shell. The magnetic measurements have revealed that a ferromagnetic (FM)-like open hysteresis exists at temperatures below the freezing point, T_F. In the high field region, a paramagnetic (PM) response appears without showing any sign of saturation. Also, the field dependent magnetization (M–H) measurement is PM-like at T>T_F. These interesting magnetic properties are shown to arise from the AFM CuO on the outer surface. They are attributed to the uncompensated surface spins of Cu²⁺ and the effect of random surface potential. More interestingly, the magnetic susceptibility is greatly enhanced in the presence of Cl⁻ anions at T<T_F, according to the field-cooled/zero-field-cooled (FC/ZFC) measurements. This further supports the point that the disorder or frustration effect of the impurity would reduce the AFM ordering of CuO and increase the level of uncompensated spins.