Synthesis of continuous spinnable carbon nanotube (CNT) fibers is the most promisingm ethod for producing CNT fibers for commercial applications. The floating-catalyst chemical vapor deposition (FC-CVD) methodi sarapid process that achieves catalystf ormation,C NT nucleation and growth, and aerogel-like sock formation within af ew seconds. However,t he formation mechanism is unknown. Herein, the progress of CNT fiber formation with bimetallic catalysts was studied, and the effect of catalystc omposition to CNT fiber synthesis and their structuralp roperties was investigated. In the case of bimetallic catalysts, the carbon source rapidly decomposesa nd generates various secondary hydrocarbon species, such as CH 4 ,C 2 H 4 ,C 2 H 2 ,C 3 H 6 ,a nd C 4 H 10 whereas monometallic catalysts generate only CH 4 and C 2 H 4 on decomposition. CNT fiber formation withF e 1 Ni 0 beginsa bout 400 mm from the reactor entrance, whereas CNT formation withF e 0.8 Ni 0.2 and Fe 0.5 Ni 0.5 begins at about 500 and 300 mm, respectively.T he formed CNT bundles and individual CNTsa re oriented along the gasf low at thesel ocations.T he enhanced rateo ff iber formation and lowering of growth temperature associated with bimetallic catalysts is explained by the synergistic effectsb etween the two metals. The synthesized CNTsb ecome predominantly semiconducting with increasing Ni contents.