A v-ribbed belt has a compression section, a tension section, and a load carrying section comprising longitudinal reinforcing cords intermediate of the compression section and the tension section. The compression section has at least one fabric layer and a plurality of longitudinally extending grooves forming longitudinally extending ribs. The fabric layer is distanced from the outermost surface of the longitudinally extending ribs and the untreated gauge of each fabric layer is in the range of 0.20 to 1.0 mm, with a preferred range of 0.25 to 0.75 mm.

A toothed belt includes a toothed rubber layer having teeth formed on a surface thereof and containing a plurality of short fibers entirely distributed therein. The surface of the toothed rubber layer, on which the teeth are formed, is covered with a cover fabric. A back rubber layer is integrally applied to the other surface of the toothed rubber layer. A plurality of cord elements is disposed across a width of the belt and is intervened between the toothed rubber layer and the back layer in such a manner that a portion of each cord element is embedded in the toothed rubber layer, and the remaining portion thereof is embedded in the back rubber layer. The short fibers are entirely distributed in the toothed rubber layer and are regularly oriented.

A power transmission belt with a good balance of high temperature resistance, good belt durability and low temperature flexibility, and a service range of about -40.degree. C. to about 140.degree. C. The main belt body portion of the power transmission belt is manufactured from an elastomeric material comprising 100 parts by weight of an at least partially hydrogenated nitrile group-containing copolymer rubber, and from about 0.5 to about 50 parts per hundred weight of said rubber of a fiber reinforcement having a length to diameter ratio of greater than 10:1. The nitrile group-containing copolymer rubber preferably comprises (1) from about 5 to about 40 percent of unsaturated nitrile monomer units, (2) from about 1 to about 80 percent of units of at least one second monomer selected from the group consisting of fluorine-free unsaturated carboxylic acid ester monomer units and fluorine-containing vinyl monomer units, or any monomer possessing the characteristic of lowering the glass transition temperature of the rubber; (3) up to about 20 percent of conjugated diene monomer units and (4) the balance being hydrogenated conjugated diene monomer units, preferably wherein the sum of the contents of the monomer units (1) and (2) is preferably from about 30 to 90 percent by weight and the sum of the contents of the monomer units (3) and (4) is from about 10 to about 70 percent by weight. The use of the fiber reinforcement in the nitrile group-containing, highly saturated copolymer rubber provides substantially improved low temperature crack resistance while improving the high temperature resistance of the belt.