Synthesis of carbon nanotubes was attempted by hot-wire chemical vapor deposition using a mixture of C2H2 and NH3 gases. Substrates used were crystalline (c-) Si and amorphous (a-) SiO2 coated with Ni films of 30 nm in thickness as a catalyst for nanotube formation. Scanning electron microscopy (SEM) images showed that micron-sized grains were present in the deposit on the c-Si substrate, whereas nano-sized grains were evident in the deposit on the a-SiO2 substrate. Nanotube formation could not be confirmed by SEM, but there is evidence of the possible formation of nanotubes on the Ni-coated a-SiO2 substrate.