Aqueous solutions of silane coupling agents used for surface treatment of glass fiber were analyzed using gas chromatography (GC), gel permeation chromatography (GPC), and field desorption mass spectrometry (FD-MS). N-Phenyl-3-aminopropyltrimethoxysilane (AnPS) and 3-amino-propyltriethoxysilane (APS) used were hydrolyzed and condensed in water-ethanol solution. FD-MS analysis was found to be suitable for the evaluation of polymerization of silanes in the process of hydrolysis and condensation in the aqueous solution. FD-MS analysis gave the molecular weight of each silane condensate to evaluate the respective molecular structure.