The purpose of this study was to examine the difference of muscle thickness of trunk between thepitching arm side and the non-pitching arm side in a baseball pitcher. Twenty college baseball pitchers( pitchers group) and 20 male college students (control group) participated in this study. Twelve musclethickness in the chest, abdomen and back were measured by using ultrasonic diagnostic apparatuswith 8 MHz probe. Muscle thickness were measured in the chest (pectoralis major, pectoralis minor), inthe abdomen (rectus abdominis, external oblique, internal oblique, transversus abdominis, quadratuslumborum ), and in the back (upper trapezius, lower trapezius, rhomboid major, latissims dorsi, erectorspinae). In the control group, there was no significant difference of muscle thickness between thedominant arm side and the non-dominant arm side in all the muscles of the chest, abdomen, and back.In pitchers group, pectoralis major and pectoralis minor muscle in the chest showed significantly largermuscle thickness on the pitching arm side, and lower trapezius and latissimus dorsi muscles in the backshowed significantly larger muscle thickness on the pitching arm side. On the other hand, the externaloblique and the internal oblique muscle in the abdomen showed significantly larger muscle thickness onthe non -pitching arm side. These results indicate that selective muscle hypertrophy on chest and backwas observed in the pitching arm side, and that on abdomen was observed in the non-pitching arm sidein baseball pitchers.