The effects of reaction temperature (873-1223 K), carbon sources (CH4, C2H2, C2H4, and C2H6), and the amount of catalyst (2.5-20 g) on the physical properties (tube diameter, conversion, volume expansion, intensity ratio of the D- and G-band peaks (I-D/I-G)) of multiwalled carbon nanotubes (MWCNTs) in a gas-solid fluidized bed reactor (with an inner diameter (id) of 0.056 in and a height of 1.0 m) have been determined. The MWCNTs synthesized by the catalytic decomposition of methane produce the smallest tube diameter and the highest intensity ratio (I-D/I-G) among the carbon sources (acetylene, ethylene, and ethane). Although the tube diameter of MWCNTs that have been synthesized from the decomposition of methane and ethane at 1073 K are similar, the volume expansion of the carbon nanotubes (CNTs) agglomerate from ethane is higher than that from methane. Both the tube diameter and the ID/IG ratio of the MWCNTs synthesized from the decomposition of methane decrease as the reaction temperature increases (in the temperature range of 1073-1223 K). The amount of catalyst does not affect the mean tube diameter of the synthesized CNTs; however, CNTs with a bamboo structure are synthesized when the carbon decomposition rate is higher than the CNT growth rate.