Carbon nanocoils (CNCs) with different diameters have been synthesized on different substrates by thermal chemical vapor deposition using Fe films as catalysts prepared by ion sputtering. It is found that CNCs with diameters greater than 100 nm are obtained as the main products in large quantities on Fe film coated indium tin oxide substrates. However, on Fe film coated SiO2 substrates, multiwall CNCs (MWCNCs) along with carbon nanotubes (CNTs) are grown, and the yield of MWCNCs is decreased rapidly with a lower Fe film thickness. The as-grown MWCNCs with observed coil diameters less than 100 nm and filament diameters less than 30 nm are much thinner than the conventional CNCs. Plate-like catalyst particles with sizes much larger than the filament diameter of the MWCNCs are observed at the roots of these MWCNCs, indicating a base growth mechanism. Furthermore, it is also observed that large particles with irregular shapes lead to the growth of helical MWCNCs, while large particles with steady circular shapes tend to grow as straight CNTs. Based on the experimental results, a growth model for MWCNCs is proposed.