The effects of the gas flow rate (2,000∼4,000 sccm), catalyst amount (10∼50 g), and static bed height (HS) of the inert particles (0∼0.10 m) on the physical properties (tube diameter, ID/IG ratio, carbon yield, and volume expansion) of multi-walled carbon nanotubes (MWCNTs) in a gas-solid fluidized bed (0.056 m-ID × 1.0 m-high) reactor were determined. Agglomerated MWCNTs were synthesized by pyrolysis of acetylene on Fe/Mo/Al2O3 catalyst in a gas-solid fluidized bed reactor. The physical properties of the MWCNTs were determined by HR-TEM, SEM, and Raman spectroscopy. The MWCNTs synthesized at 873 K had an outer diameter of a few tens of nanometers. The MWCNTs produced from the inert medium fluidized bed reactor were entangled sub-aggregates with an average diameter of 15∼20 nm. Although carbon nanotubes (CNTs) could not be synthesized well at a very low gas velocity, they could be synthesized well when inert medium particles were added into the reactor, and the average CNT diameter varied with respect to the gas velocity. Volume expansion of the synthesized CNTs after completion of the reaction decreased with increasing HS. However, the carbon yield of acetylene was not significantly changed with increasing HS. Volume expansion of the CNT agglomerates and the carbon yield of acetylene decreased with increasing catalyst amount.