Ni-assisted thermal chemical vapor deposition (TCVD) is one of the most common techniques for the growth of carbon nanofibres/nanotubes (CNFs/CNTs). However, some fundamental issues related to the catalytic growth of CNFs/CNTs, such as the low-limit growth temperature, the limiting steps and the state of Ni, are still controversial. Here, we report the growth of CNFs at 300 °C; that is the lowest temperature for the growth of CNFs by TCVD using Ni as the catalyst so far. The results showed that the Ni existed in rhombohedral Ni3C, not in the normal form of face-centered cubic Ni, and the C atoms for building the CNFs were precipitated from the (001) planes of the faceted Ni3C nanoparticles. The CNFs are believed to be formed by the decomposition-formation cycle of metastable Ni3C that has a low-limit decomposition temperature of about 300 °C. Our results strongly suggest that TCVD is a valuable tool for the synthesis of CNFs/CNTs at temperatures below 400 °C, which is generally considered as the upper-limit temperature for fabricating complementary metal oxide semiconductor devices but is the low-limit temperature for growing CNFs/CNTs by TCVD at present.