The effects of the metal loading (30-70 wt.%), metal molar ratio (Co/Fe, 1-5) and mass ratio of citric acid to the catalyst (0-0.6) on the productivity and mean diameter of the multi-walled carbon nanotubes (MWCNTs) in a gas-solid fluidized bed reactor (with an inner diameter of 0.056 m and a height of 1.0 m) were determined. Liquefied petroleum gas (I.PG) was used as the carbon source. X-ray diffraction (XRD) was used to characterize the catalysts synthesized using a combustion method. MWCNTs synthesized in the fluidized bed reactor were characterized by field emission scanning electron microscopy (FE-SEM) to observe their morphologies and measure their diameters. Productivity was increased by increasing both the metal loading and the mass ratio of citric acid to the catalyst. A high productivity, up to 2000%, was obtained. The catalyst transition metal particle grain size decreased in the range of 8-17 nm with an increasing citric acid mass ratio to the catalyst and the mean diameter of the MWCNTs decreased with increasing the metal molar ratio, however the correlation between the grain size in the catalyst and the mean diameter of MWCNTs remains unclear. Crown Copyright (C) 2009 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.