Ru catalyst confined within the channels of multiwall carbon nanotubes (MWCNTs) is prepared by opening tube ends via mixed concentrated acid oxidation or catalytic oxidation, followed by filling via wet impregnation. The catalyst filling ratio is characterized by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The defects and functional groups are detected and quantified by Raman, fourier transformer infrared spectroscopy (FTIR) and XPS. The results show that high catalyst filling efficiency (similar to 80%) can be achieved with catalytic oxidation pre-treatment. The acid oxidation process can introduce more oxygen-containing functional groups such carboxyl and phenol groups. Both FTIR and XPS provide evidence for functional groups removal during thermal treatment. The effects of the nanotube length distributions, defects and functional groups on filling efficiency have been discussed intensively. (C) 2011 Elsevier Ltd. All rights reserved.