In order to investigate the reaction of silane-treated glass fiber with thermosetting resin, a set of spectra were measured near the interface between the glass fiber and vinyl ester resin using microscopic FT-IR spectroscopy. A thin-sliced sample was made from glass fiber/vinyl ester resin composite to be subjected to the spectroscopy. The spectra were obtained through a 20 x 40 mum of a mask window at each 20 mum step from the glass strand area to the matrix resin. The intensity at 1450 cm(-1) of the spectra decreased from the glass strand area to the matrix resin. It was noticed, however, that the intensity kept the same value even in the matrix at a distance of 80 mum from the glass strand area. This meant that the resin in such an area was denatured by the reaction of silane to resin at the surface. This mechanism was supported by the significant size of the denatured area, which increased with amounts of silane at the surface of glass fiber.