By simple impregnation and hydrothermal treatment, MgO supported iron catalysts were obtained and used for carbon nanotube (CNT) growth from chemical vapor deposition with methane as the carbon source. Single/double/multi-walled CNTs (S/D/MWCNTs) were selectively synthesized on the Fe/MgO catalyst with different iron loadings. When the iron loading was low (0.5%), the iron atom distributed on the MgO support was sintered to iron nanoparticles with a size of 0.8-1.2 nm under the growth conditions. This catalyst promoted the formation of SWCNTs, which was attributed to the surface diffusion of carbon atoms on it. The selectivity for SWCNTs in the as-grown product from the 0.5%Fe/MgO catalyst was 90%, and the carbon mass yield was 19 times that of the active phase. When the iron loading was increased to 3%, larger iron catalyst particles of about 2.0 nm were formed. On this catalyst, there was more bulk diffusion of carbon, and DWCNTs became the main products due to the combination of both surface and bulk diffusion. With the iron loading was further increasing, iron particles from 1 to 8 nm were formed, which promoted the growth of MWCNTs together with S/DWCNTs. With increasing iron amount on the porous MgO support, the diameter, wall number, and proportion of semiconducting CNTs also increased. This provides a controllable way to selectively grow S/D/MWCNTs on a large scale in a fluidized bed to meet critical needs for CNTs in applications.