We have synthesized multi-walled carbon nanotubes (MWNTs) on nickel-deposited n-type silicon substrates by thermal chemical vapor deposition (Thermal CVD). The electroless plating method was adopted in the deposition of the Ni catalytic layer. Sulfuric acid solution was used as a buffer to adjust and maintain a pH value of 4.6. Both the deposition time of the nickel catalyst layer and the growth time of MWNTs were adopted to control the length and density of MWNTs. Scanning electron microscopy (SEM) images showed that the density of carbon nanotubes increased as the deposition time of nickel catalyst layer increased. The formation of nickel nucleation becomes rich as the immersion time of the substrate in electroless plating solution was longer, and this benefited the growth of carbon nanotubes. In addition, the Raman spectrum demonstrated that the ID/IG ratio of MWNTs decreases as the deposition and growth time increase, indicating that more graphene MWNT structures were formed.