Abstract: An endless power transmission belt having a plurality of grooves which are formed in the inner surface of the belt in such a manner that the grooves extend perpendicular to the longitudinal axis of the belt, and a plurality of cogs each of which is formed between adjacent grooves thereof. At least two of three factors, groove pitch in the longitudinal direction of said belt, grooves depth and groove cut angle are changed at random. In one example, the groove pitch is changed at random in a range of 60% to 100% of the thickness of said belt, groove depth is changed at random in a range of 20% to 40% of the thickness of the belt, and the groove cut angle is changed at random in a range of 10.degree. to 40.degree..