The timing belt and a coupling structure of the same are disclosed, in which a stress focused at a corner portion decreases by forming an optimum shaped groove at a corner portion of a timing belt, so that it is possible to largely increase a fatigue life span of a timing belt and a durability of a timing belt. In addition, noises and vibrations can be minimized by forming a through hole at a threaded surface of a timing belt or a threaded portion of a sprocket. In the timing belt, which has threads at a belt body, there is provided an improved timing belt comprising a groove formed at a corner portion in which the threads and the belt body contact with each other.