A power transmission belt comprises a compression rubber layer and an adhesion rubber layer both of which are formed of vulcanizate of an ethylene-a-olefin-diene rubber compound, with a preferred ethylene content of 55-85% in the compression rubber layer. A plurality of load carrying cords formed of fiber cords adhere to and embed in the adhesion rubber layer. The compression rubber layer has a modulus of elasticity in tension of a in the lengthwise direction of the belt and a modulus of elasticity in tension of b in the widthwise direction of the belt, and a and b satisfy the relationship: 0.9≦a/b≦1.2. The compression rubber layer may contain short fibers therein in an amount of not more than 5 parts by weight in relation to 100 parts by weight of the ethylene-a-olefin-diene rubber, used in such a manner that the fibers are oriented in the lengthwise direction of the belt.