The common disorder called tennis elbow exhibits typical clinical characteristics, i.e. painful condition at the lateral aspect of elbow joint on resisted wrist extension. However an exact cause for this painful condition has not yet been established. Many observers believe that the usual lesion of tennis elbow is a partial rupture of the extensor tendon at the tenoperiosteal juction on the lateral epicondyle of humerus. However the mechanism of the tendon rupture has never been explained. Conservative treatments on the tender area have been the most common therapeutic modalities for pain relief of tennis elbow. Based on my clinical experiences and anatomical studies, I discerned that tennis elbow is a periostitis of lateral epicondyle of humerus secondary to spastic contraction of muscular belly of extensor carpi radialis after over-stretched injury. Therefore, spasmolytic treatment on the extensor carpi radialis muscle provided a favorable result for permanent relief for tennis elbow pain.