To provide a high-tenacity glass fiber cord for rubber-reinforcement, coated with a treating agent having sufficient heat-resistance and water- resistance and resistant to the lowering of strength even by using under a condition subjected to high load and repeated flexural stress at a high temperature and provide a rubber belt produced by using the cord.

This fiber cord is produced by paralleling 1-10 strands each composed of 200-2,000 high-strength glass fiber filaments having a diameter of 3-6 μm and collected together by a collecting agent to obtain an under-yarn containing 500-5,000 filaments and having a fineness of 50-150 tex, covering the surface of the yarn with a coating layer containing a resorcinol- formaldehyde condensate and a vinylpyridine-styrene-butadiene terpolymer and chlorosulfonated polyethylene or a nitrile-containing highly saturated polymer latex, subjecting the under-yarns to primary twist, doubling 2-20 coated and twisted under-yarns and subjecting the doubled yarn to final twist.