One of the keys to understanding the growth mechanism of carbon nanotubes (CNTs) is the influence of the oxidation states of the iron catalyst nanoparticles. We examined the effect of the oxidation states of iron nanoparticles, which could be controlled by varying the electrical conductivity of the silicon substrate, on the growth of CNTs during thermal chemical vapour deposition. Since the thermal conductivities of the silicon substrate is proportional to its electrical conductivities, thereby catalytic iron thin film on the silicon substrate with higher thermal conductivity could be more affected at the same temperature. The p-type silicon substrates with the electrical conductivities of 1.7 x 10(-3)/Omega cm (substrate U) and 2.8 x 10(2)/Omega cm (substrate H), respectively, were used in this work. The substrate H allowed that the oxidized iron was more reduced to metallic iron and iron particles grew increasingly larger during annealing under NH3/Ar environment at 900 degrees C. This size increase is due to the tendency of the reduced metallic iron particles to agglomerate on the substrate. By showing that the oxidation states of iron catalyst could be controlled by the electrical conductivities of the silicon substrates, resulting in the evolution of catalytic nanoparticles for CNTs growth, the electrical conductivities of the substrate could be used as a key parameter for understanding CNTs growth mechanism. (C) 2015 Elsevier B.V. All rights reserved.