In this work, multi-walled carbon nanotubes (MWCNTs) with significantly different mean diameters were produced by catalytic CVD over Ni and Co-based supported catalysts. Our results indicate that Ni nanoparticles in the Ni/MgO catalyst are responsible for controlling the inner diameters of the carbon nanotubes. Contrary, Co nanoparticles in the Co/MgO catalyst control the outer diameters of MWCNTs. The “base-growth” mechanism and smaller diameters of the MWCNTs grown on the Ni/MgO catalyst are associated with a strong metal-support interaction (SMSI) resulting from NixMg1−xO mixed oxide formation. The concept of the weak metal-support interaction (WMSI) between Co nanoparticles and MgO for the Co/MgO catalyst confirms the “tip-growth” mechanism of the MWCNTs.