Using Ni/MgO catalyst,carbon nanotubes(CNTs) were synthesized on silicon by radio-frequency plasma enhanced chemical vapor deposition(RF-PECVD) method.The morphologies and microstructures of the CNTs were characterized by X-ray diffraction(XRD),scanning election microscopy(SEM),Raman spectroscopy and high-resolution transmission electron microscopy(HRTEM).The results show that as the MgO content increases,carbon nano-onions transform into CNTs,the nanotube diameter becomes smaller and uniformity,the purity of CNTs is improved,and crystallinity is better;The top of the CNTs transforms open or closed carbon nano-onion structure,and in the backbone of the tube area,there exists cataly.With the increase of MgO content,the nano-particles change into nanowire.When the MgO content is 50%,the outer wall of part CNTs forms a single-layer graphite,its length is 7 nm,and the CNTs are filled with Ni nanowires whose its length is 60 nm.