The effect of substrate positions inside the chemical vapor deposition reactor on the length and quality of the grown carbon nanotube (CNT) arrays is reported. It was found that longer CNT arrays are grown when located downstream on the platform in the reactor. This effect becomes more pronounced for increased growth time. Related factors such as temperature of the gas mixture and its flow velocity seem to be responsible for the behavior. The quality of the CNTs is not affected by the position of the substrates inside the reactor.