The purpose of this study was to compare the thickness of the abdominal muscles during single leg holding exercise (SLH) in a hooked lying position on stable surface and on a foam roller. Healthy twenty subjects who had no medical history of lower extremity or lower back pain were recruited for this study. Muscle thickness of transverse abdominis (TrA), internal oblique (IO), and external oblique (EO) was recorded using real-time ultrasonography during SLH. Paired t-test with Bonferroni adjustment was used to compare the muscles thickness during SLH on stable surface and on a foam roller. The result showed that TrA and IO muscle demonstrated greater thickness during SLH on foam roller than those on the stable surface. This finding suggests that SLH on an unstable foam roller is more effective to increase thicknesses of TrA and IO muscles than stable surface.