Results of thorough studies of the catalytic synthesis of carbon nanofibers (CNFs) by the decomposition of ethylene using Y zeolite as the support and Ni as the active phase were discussed. Experimental results clearly indicated that the metal-incorporation method (ion exchange or impregnation) had very significant effects not only on CNFs growth but also on the deactivation rate, the final yield of CNFs, and the characteristics of the synthesized CNFs. CNFs synthesized from the impregnated catalyst grew from small and well-dispersed Ni particles anchored to the outer surface of the zeolite. Nevertheless, CNFs synthesized from the ion-exchanged catalyst grew from Ni particles (of very small size) lodged inside the pore system of the zeolite. Reaction temperature and C2H4/H-2 (v/v) had a considerable effect on both carbon yield and CNFs morphology. Two types of CNFs were observed as a function of the reaction temperature: "fishbone structures" at temperatures below 600 degrees C and "tubular structures" at temperatures above 600 degrees C. On the other hand, as the C2H4/H-2 ratio was decreased, the CNFs became slightly more graphitic in nature and the arrangement of graphite sheets changed from the fishbone structure to "octopus carbon".