Carbon nanotubes (CNTs) were prepared on Si substrate by radio frequency plasma-enhanced chemical vapor deposition with Ni/MgO as catalyst. The morphology and microstructure of CNTs were characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. The results show that with increasing MgO concentration in the catalyst the diameter of CNTs becomes smaller and more uniform and their purity and crystallinity are improved. Open or close carbon nano-onion structure is formed at the top of the CNTs, and the catalyst particles are found inside the tubes. With increasing MgO concentration, nanoparticles tend to nanowires longer than 30 nm for MgO concentration of 75% in the catalyst compund. These nanowires are made by Ni. Besides, when MgO concentration is 50%, the graphite layer of the bending tube wall extends inside of the tube, dividing the cavity into several irregularly shaped spaces, which forms a new carbon nanostructure.