The direct spinning process is a useful method for continuously synthesizing carbon nanotube (CNT) fibers. A reactor should be designed based on reactor design principles in order to study the synthesis mechanism and mass production of CNT fibers. The internal temperature in a continuous flow reactor depends on the length of the reactor, and because the temperatures required for the decomposition of various feedstocks differ, each feedstock is decomposed at a different point in the reactor. Here, we report that high-quality CNT fibers were continuously synthesized by water injection at temperatures above the catalyst-forming range during the direct spinning process, as determined by the decomposition temperatures of the feedstocks. The decomposition temperatures of the feedstock materials were measured. Thiophene and toluene decomposed at 500-600 degrees C and 600-700 degrees C, respectively, and these temperatures were lower than those reported in the literature. The quality of the CNT fibers was improved by injecting water into zones with temperatures that exceeded the decomposition temperatures of the feedstocks in the reactor. Oxygen containing functional groups formed less often on the CNT surfaces, further improving the quality of CNT fibers. Moreover, the CNT fibers were continuously synthesized, even if excess water were injected. (C) 2018 Elsevier Ltd. All rights reserved.