A series of water-soluble silicon oxide materials was synthesized by the sol-gel reaction of tetramethoxysilane (TMOS)/3-aminopropyltrimethoxysilane (APTMOS) mixtures catalyzed by hydrochloric acid with varying TMOS/APTMOS ratios. Increase in TMOS composition made the products less water-soluble, and the richest TMOS composition for the water-soluble silicon oxide materials was found to be TMOS/APTMOS = 7/3 (mol/mol). Ultraviolet-visible spectroscopy of a 10 wt% aqueous solution of the product with TMOS/APTMOS = 7/3 showed a high transmittance in the visible region. Its infrared spectrum showed an absorptionband attributable to the Si–O bond, and the 29Si dipole decoupled/magic angle spinning–nuclear magnetic resonance spectrum exhibited signals in the regions of T3, Q3, and Q4, indicating a dense siloxane network structure. A peak due to an ordered structure with a 1.76 nm periodicity was observed in the x-ray diffraction profile, and a stripe pattern was observed in the transmission electron microscopy image. These results indicate that the rodlike silicon oxide macromolecules with a 1–2-nm diameter stack parallel. Thus a silicon oxide material with nano-ordered structure was successfully formed and such a structure produced the water solubility.