The purpose of this study was to determine the differences in the growth patterns of the external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) muscles in adolescence baseball players. The subjects were 17 elementary school students (ES), 56 junior high school students (JS), and 21 high school students (HS) who played baseball. The EO, IO, and TrA muscle thicknesses were imaged using an ultrasound imaging device and analyzed separately on the dominant and non-dominant sides. Curve regression estimation of the relationship between EO, IO, and TrA and chronological age was used to read inflection points. An analysis of covariance was also performed, using body weight as a covariate for comparison with each of the EO, IO, and TrA grades. The inflection point readings showed that TrA was 11.26 years old (p<0.01, R2=0.45) for the dominant side and 10.10 years old (p<0.01, R2=0.46) for the non-dominant side, earlier than the inflection points of EO and IO. Analysis of covariance showed no significant difference in EO between any of the grades. IO showed significantly higher values for muscle thickness as the grade increased, both on the dominant and non-dominant side. TrA was higher for JS among ES and JS and higher for HS among ES and HS on both the dominant and non-dominant sides. EO may growth with increasing body size, while IO is interpreted to increase with increasing age due to the movement of baseball. TrA may grow earlier than EO and IO.