The purpose of this study was to determine the effect of mental practice on biceps brachii muscle strengthening in 3 poststroke hemiplegia patients along with multiple baseline design across subjects. The mental practice adopted for this experiment involved imagery training to practice elbow flexion, which last 12 to 16 sessions with 30 minutes each session. Maximal muscle activities was measured pre- and post-mental practice to evaluate the strength of biceps brachii. The strength of biceps brachii was measured by surface-EMG. The results were: In the subject 1, 73.92 mV (pre-) and 127.56 mV (post-); in the subject 2, 147.60 mV (pre-) and 202.85 mV (post-); and in the subject 3, 20.75 mV (pre-) and 27.92 mV (post-). The results indicate that a simple mental practice is a useful method to strengthen biceps brachii muscle in hemiplegic patients.