The effect of operating variables (reaction temperature, space velocity, and partial pressure of the carbon source) on carbon yield was determined for synthesizing MWCNTs by catalytic chemical vapor deposition using ethylene as the carbon source in a batch type fluidized bed reactor. Stable fluidization was maintained using the MWCNT agglomerate as the initial bed material and MWCNTs with high selectivity were synthesized. The optimal reaction temperature for the maximum carbon yield was 983 K and the apparent activation energy was 102 kJ/mol. The maximum carbon yield occurred at the space velocity of 136 h(-1). The carbon yield increased with increasing partial pressure of ethylene. However, the synthesis reaction of MWCNTs did not proceed at partial pressures above 0.63. A correlation is proposed to predict the yield of MWCNTs. (C) 2015 Elsevier Ltd. All rights reserved.