Carbon nanotubes (CNTs) with tunable density have been grown directly by inductively coupled plasma chemical vapor deposition (ICP-CVD) using anodic aluminum oxide (AAO) on silicon as a substrate. The density of CNTs (10(6) to 10(9) CNTs/cm(2)) can be controlled by changing the length of catalytic Ni nanowires embedded in AAO. The one-dimensional CNTs-Ni nanowire heterojunction without observable defects by transmission electron microscopy characterization ensures good contact between the CNT and the Ni nanowire. The realization of combining AAO on silicon and controlling the density of carbon nanotubes by a plasma process is significant for application of CNTs as field emitters and nanoelectronic devices. (C) 2004 The Electrochemical Society.