PURPOSE: To obtain inorganic yarn for fiber reinforcing, having improved adhesiveness to a matrix resin and reduced friction between fibers to facilitate opening, causing no fluff by providing inorganic yarn with a sizing agent comprising specific components.

CONSTITUTION: (A) 100 pts.wt. ester comprising 16-20C higher unsaturated fatty acid and 6-20C higher unsaturated alcohol is blended with (B) 10-50 pts.wt. adduct of 6-22C higher hydroxy acid triglyceride with 7-9mol ethylene oxide, (C) 5-15 pts.wt. adduct of 6-22C higher hydroxy acid triglyceride with 10-15 mol ethylene oxide and (D) 10-25 pts.wt. adduct of 8-18C higher alcohol with 8-15 mol ethylene oxide to prepare an aqueous dispersible sizing agent. Since the coefficient of friction of inorganic fibers is reduced when coated with the sizing agent in 0.01μm thickness, opening of yarn bundle is facilitated, fluff does not occur and adhesiveness to a matrix resin is improved and inorganic yarn coated with the sizing agent is suitable as reinforcing fibers for composite materials.