Multi-walled carbon nanotubes (MWNTs) have been successfully modified with CuO, Cu 2 O and Cu nanoparticles via a simple method, and the calcination temperature, the amount of NH 3 ·H 2 O and soaking time play critical roles in controlling the final products. The modified MWNTs have been characterized by X-ray diffraction, infrared spectrum, scanning electron and transmission electron microscopies. Optical absorption of the obtained products has also been investigated, and the quantum confinement effect was illustrated in the absorption spectra.