Abstract: To measure the thickness of Transversus Abdominis (TrA), Internal Oblique (IO), External Oblique (EO) and Multifidus (MF) and compare the effects on trunk stability according to the performing abdominal drawing-in manuever during plank exercises. Thirty healthy male and female adults were recruited. Subjects perform the plank exercises after measuring thickness of TrA, IO, EO and MF in resting position. When performing the General Plank(GP), General Plank with Abdominal drawing-in maneuver (GPA), Unstable Surface of Plank (USP) and Unstable Surface of the Plank with Abdominal drawing-in maneuver (USPA), we measured three times and used the value of measurements. All measurements were used in one way repeated ANOVA. The results of study, resting time and each plank exercises showed significant differences in TrA, IO, MF muscles. In the comparison between each plank exercises, the TrA, IO, MF activation increased because that muscle were thicker than other plank exercises when performing GPA and USPA. These findings suggest that a plank exercise with abdominal drawing-in maneuver is useful way in trunk stability.