The effect of catalyst support and reactant on the yield and structure of carbon growth has been investigated in the chemical vapor deposition (CVD) process. Powder Fe and Fe/Al2O3 were the catalysts studied, and CO/H-2, CO, CH4, and C2H6/H-2 were used as gas precursors. Platelet and fishbone-tubular structures were produced on powder and supported Fe from CO/H-2, with average diameters of 115 and 45 nm and yields of 28.8 and 17.6 g of C/g of cat. in 8.5 h, respectively. Onionlike carbon was the main structure produced from pure CO on both catalysts. In contrast, from hydrocarbons the highest yield of 2.24 g of C/g of cat. was achieved on Fe/Al2O3, with predominantly tubular structures produced and average tube diameters close to 21 nm. It is concluded that the reactivity and carbon nanostructures are dictated by the size and crystallographic orientation of the catalyst particles. It has been suggested that the tubular structures were grown by continuous carbon supply directly to the tube, but the fiber structures were grown in a layer-by-layer manner. Controlled synthesis of carbon nanotube, platelet nanofiber, fishbone-tubular nanofiber, and onionlike carbon with high selectivity and yield was demonstrated.