Investigations of carbon nanotube (CNT) synthesis were carried out in a chemical vapor deposition reactor using CO as the carbon source, Fe as the catalyst material, and SiO2 as the catalyst support. This allowed us to synthesize CNTs in a wide range of diameters from 1.4 to 12 nm and lengths from 0.5 to 350 μm. An incremental variation in the number of CNT walls, increasing from one to four, was found in the temperature range of 590−1070 °C. The increase in CNT wall number and CNT length with temperature can be explained by enhancement of carbon solubility and diffusivity. CO2 was found to be a very important additive for the activation of catalyst particles and subsequently for the successful synthesis of CNTs. The prevention of cementite particle formation and etching amorphous carbon were also attributed to CO2.