Glass fiber-treating agent which includes: (A) 5-95% by weight of an acetylene glycol or an ethylene oxide and/or a propylene oxide adduct of an acetylene glycol; and (B) 5-95% by weight of a silicone surfactant which may be polyoxyalkylene-modified. The glass fiber-treating agent reduces dynamic and static surface tension and contact angle. The glass fiber-treating agent provides excellent wetting, penetration, and antifoaming properties when glass fibers are treated therewith.