The invention provides a glass fiber for reinforcing rubber product which is excellent in the impregnation of an RFL treating agent to a glass fiber strand, has less blister of the coating layer made of the RFL treating agent, has excellent appearance and physical performance, and has little fluctuation of quality; and a method for producing such a glass fiber. An RFL treating agent comprising, as a main components, a rubber latex and a water-soluble condensate of resorcinol with formaldehyde, is impregnated to a glass fiber strand having from 200 to 2,000 glass filaments bundled, so that it is impregnated to each glass fiber strand independently without drawing such glass fiber strands together, and then, the RFL treating agent impregnated to the glass fiber strand, is solidified to forma coating layer, to obtain a coated glass fiber. Then, the coated glass fiber is subjected to twisting to obtain a twisted yarn, and at least two such twisted yarns are put together and subjected to plying, to obtain a glass fiber for reinforcing rubber products.