[Purpose] The purpose of this research was to determine the effect of Flexi-Bar exercise with vibration on trunk muscle thickness and balance in university students in their twenties. [Subjects and Methods] This research evaluated 26 university students in their twenties, equally and randomly divided into two groups. Both the experimental and control groups used an ordinary pole for exercise. In addition, the experimental group exercised by using a Flexi-Bar. Ultrasonic imaging was used to measure the changes in trunk muscle thickness. A balance measuring equipment was used to measure balance ability. [Results] The thickness of the transversus abdominis and the multifidus muscles in the experimental group increased, and the experimental group showed increased thickness in the transversus abdominis muscle compared to the control group. After 6 months of exercise, there was an improvement in the blind Romberg test and center of pressure moving distance with one-leg standing. [Conclusion] These results indicate that the Flexi-Bar exercise is effective in increasing trunk muscle thickness and improving balance.