The formation processes of the silica nanotubes by a surfactant-assisted templating mechanism were elucidated in a laurylamine hydrochloride (LAHC)/tetraethoxysilane (TEOS) system by measuring the evolution of the shape and size of surfactant/silicate molecular assemblies with small-angle X-ray scattering, together with TEM, SEM, nitrogen adsorption isotherm, and Si-29 NMR. The formation processes in dilute LAHC solution at pH 4.5 are elucidated as follows. Partially hydrolyzed TEOS penetrates into finely divided bilayerlike surfactant assemblies and converts the assemblies to globular aggregates. Then, the polymerization of the hydrolyzed TEOS proceeds on the surface of the aggregates, converting the aggregates to cylindrical form due to the geometric matching between the occupied area of a surfactant molecule and that of four SiO2 molecules at the surface of the cylinder. Then, the cylinders connect with each other at the cylinders spherical end cap because of the relative instability at the end caps due to the lack of geometric matching. Some long cylinders combine and generate bundles composed of a range of several to dozens of fine silica nanotubes. The long bundles make a network structure by cross-linking, and the solution becomes a gel. The important factors controlling the microstructure formation are (1) geometric matching of the occupied area between the silicate molecules and the surfactants at the interface and (2) the packing parameter of the silicate/surfactant assemblies. The elucidated formation processes were confirmed by the formation of long length bundles of nanosize tubules with the aid of trimethylsilylation.