Nylon-6 substrates were coated with SiO2-RSiO(3/2) (R = methyl, vinyl, phenyl) thin films by the sol-gel method, and their water permeability was evaluated. Tetraethoxysilane (TEOS) and trifunctional alkoxides such as methyltriethoxysilane (MTES), vinyltriethoxysilane (VTES), and phenyltriethoxysilane (PhTES) were used as starting materials. Water permeability coefficients of the nylon-6 substrates coated with these SiO2-RSiO(3/2) thin films were increased with an increase in the content of these trialkoxysilanes in the films. The water permeability coefficients of these coated, nylon-6 substrates were smaller in the order of the systems TEOS-PbTES < TEOS-VTES < TEOS-MTES in the relatively low content of the trialkoxysilanes.