The density control of vertically aligned carbon nanotubes (CNTs) has been studied using a thermal chemical vapor deposition (CVD) method in correlation with the thicknesses of the Fe/Al multilayer catalyst on a Si substrate. The density of CNTs could be controlled through the thicknesses of both the Al layer and Fe catalyst layer. The low density of the Fe catalyst grains formed by thermal treatment, caused the formation of CNTs with random orientations, and resulted in a decrease in the CNT height. The optimum film thickness of the Fe/Al multilayer catalyst for obtaining long CNTs (220 um) was determined for the case of a substrate temperature of 700 °C. © 2006 The Japan Society of Applied Physics.