Large amounts of amorphous carbon nanotubes (ACNTs) were prepared with Co–Ni alloy powders as catalyst in hydrogen gas atmosphere by a modified arc discharging furnace which can control temperature during the electric arcing process. The experimental results indicate that the cooperative function of temperature and catalyst plays an important role in the soot production rate and the relative ACNT purity. When temperature increases from 25°C to 700°C, the soot production rate increases from around 1 g/h to 8 g/h, the best relative ACNT purity at 600°C can reach up to 99% compared to the room temperature sample. Without catalyst, only plate graphite is formed at 25°C and very few carbon nanotubes are found when temperature increases to 600°C. TEM, SEM, HRTEM and XRD analysis showed that the as-prepared carbon nanotubes are almost amorphous. The soot production rate is 8 g/h and diameter range of amorphous carbon nanotubes is about 7–20 nm, respectively. © 2005 Elsevier Ltd. All rights reserved.