The purpose of the current study was to determine the intra- and inter-rater reliability of muscle thickness (MT) measurement of the psoas major (PM) using ultrasonography (US) conducted at different inward pressures of approximately .5 ㎏, 1.0 ㎏, 1.5 ㎏, and 2.0 ㎏. Twelve healthy male subjects were recruited for the study. The thicknesses of both PMs of each subject were measured by two different examiners in a random manner to assess the intra- and inter-rater reliability. The measurement values were analyzed using the intra-class correlation coefficient (ICC) with a 95% confidence interval (CI). ICC (2,1) was used to determine the inter-rater reliability and ICC (3,1) was used to assess the intra-rater reliability of the MT measurement of the PM. The results indicated higher ICC values for intra-rater reliability compared to inter-rater reliability. In addition, the value for intra-rater reliability with .5 ㎏ inward pressure [ICC=.99 (95%CI=.98∼.99)] was higher compared to 1.0 ㎏, 1.5 ㎏, and 2.0 ㎏. Other inward pressures for intra- and inter-rater reliability in current study were also demonstrated to have excellent values (ICC=.94∼.99). These findings showed that maintaining consistent inward pressure is essential for maintaining reliability of the results when the MT of the PM is measured by different examiners in a clinical setting.