The aim of this study was to investigate muscle activation characteristics of the upper arm during the tennis two-handed backstrokes. Eight skilled female tennis players were participate in this study. Muscle activity was determined using surface electromyographic (EMG) techniques, and the critical instants of acceleration phase were determined using high-speed video cameras. The EMG activity was recorded from four upper limb muscles (biceps brachii and triceps brachii of both right and left arms). EMG rms values of the burst of different phases were individually calculated. The results of this study were summarized as followed: The muscle activation of biceps brachii and triceps brachii muscle during acceleration phase was significantly larger than back swing and follow-through phase for both arms (p < .05). The muscle activation during back swing phase was smaller than other phase; acceleration phase was larger than other phase. The activities of the triceps brachii were greatest during backhand strokes. Tennis players should strengthen these muscles in order to prevent injury.