Carbon nanotubes have been grown on Ni-coated Si substrates by microwave plasma-enhanced chemical vapor deposition with a mixture of methane and hydrogen gases at temperatures ranging from 520 to 700 degrees C. The density and the length of the carbon nanotubes increased with increasing growth temperature. At a growth temperature of 520 degrees C, the carbon nanotubes were curly, whereas the nanotubes were straight and self-aligned upward at temperatures above 600 degrees C. Images from high-resolution transmission electron microscopy showed that the nanotubes were multiwalled, with a few wall structures. The graphitized structures were also confirmed by Raman spectra. We show that the size of Ni grains on Si substrates is correlated to the diameters of the grown carbon nanotubes.