A nucleation model was proposed for a carbon nanotube enclosing a Ni bar which was grown by chemical vapor deposition (CVD) at 700 °C using round Ni particles. At an early stage of CVD, each round Ni particle with a diameter of about 30 nm is covered with graphite layers. The graphite-covered Ni particle is considered to be unstable because the graphite layers have a large curvature. This instability is thought to make the graphite-covered Ni particles transform into a Ni bar enclosed within a carbon nanotube. In order to verify this nucleation model, we show that the size of the round Ni particle is a decisive condition for carbon nanotube formation by CVD, and that an intermediate state of the transformation of the graphite-covered Ni particles to the carbon-nanotube-enclosed Ni bar was observed by transmission electron microscopy. ©1997 American Institute of Physics.