The present paper reports the mechanism of nanotube growth and the effect of NH4F on the nanotube formation on a pure titanium substrate. A set of experiments was performed under different polarization times (ranging from 10 min to 4 h) and different NH4F concentrations of anodization. The mechanism of nanotube growth can be explained as follows. The nanoporous structure firstly formed on the pure titanium after a threshold time (0.5 h-1 h). Then the nanopores converted into self-organized nanotubes. The "shut down" tubes were found in the cross-sectional SEM images of the nanotubes due to the insufficient fluoride ion concentration. Furthermore, a possible physical model was also reported to explain the function of fluoride ions in the formation of nanotubes. (C) 2011 Elsevier B.V. All rights reserved.