Synthesis of multi-walled carbon nanotubes (MWCNTs) was accomplished by catalytic chemical vapor deposition of ethylene over Co/Al2O3 in a fluidized-bed. The reaction temperature and ethylene concentration, as the molar percentage (mol%), were both found to be crucial factors determining the solid carbon conversion level and selectivity of MWCNT formation, but had no significant effect on the size distribution of the obtained MWCNTs. Amorphous carbon and carbon nano fibers (CNFs) were the main products obtained at a reaction temperature of 550 degrees C. Amorphous carbon was also formed when using ethylene at a high concentration (62.5 mol%), which possibly deactivated the catalyst. Increasing the reaction temperature from 550 to 650 degrees C resulted in better graphitized MWCNTs. The average diameters of the synthesized MWCNTs were in the range of 7-8 nm independent of the reaction temperature or ethylene concentration. The selectivity of alkane production decreased considerably at reaction temperatures above 675 degrees C, resulting in a higher productivity of MWCNTs. The activation energy for MWCNT formation was found to be 65.3 kJ/mol, which matched well with that previously reported for carbon diffusion in liquid cobalt.