Background : We aimed to determine the effect of a mental practice on the electromyography (EMG) activity of the quadriceps femoris muscle of the affected side in a hemiplegic patient during sit-to-stand and stand-to-sit tasks. Methods : We recruited a male patient who had suffered from right hemiplegia. A single-subject experimental A-B-A design was used. The reliability of visual analysis, which was primarily employed in this study design, was supported by using the "2- standard-deviation band" method. The target motions consisted of two activities: sit-to-stand and stand-to-sit. The EMG measurements of the quadriceps femoris muscles of the affected and unaffected sides were made in 8 sessions in the baseline (A), mental-practice (B), and follow-up (A) phases. During the mental practice phase, the 10-min mental practice was performed, and the mental practice was withdrawn in the follow-up phase in order to identify the carry-over effects of the treatment. Data was calculated as the percentage of the ratio of the quadriceps EMG activity values of the affected side on unaffected side. Result : The percentage EMG-activity ratio showed a greater increase in the mental-practice phase than in the baseline, and it was maintained during the follow-up phase. Most of the data points in the mental-practice and follow-up phases were positioned above the "2-standard-deviation band." Conclusion : These findings suggest that mental practice can be used to improve the physical function of hemiplegic patients.