Purpose: Most studies have reported that the abdominal muscle thickness differs according to gender but none of these studies reported a gender difference in the thickness of the multifidus and erector spine. The spinal alignment is affected by the left and right balance in the trunk muscle. The aim of this study was to identify the trunk muscle symmetry according to gender and the correlations of the trunk muscle thickness with spinal alignment. Methods: Forty three subjects(27 males and 16 females) were enrolled in this study. The trunk muscle thickness was measured by ultrasonography. The trunk muscle, which consisted of the rectus abdominis (RA), external oblique abdominis (EOA), internal oblique abdominis (IOA), transverse abdominis (TrA), erector spine (ES), and multifidus (MF), was measured. The spinal alignment was measured by Formetric-III 3D analysis. The dependent variables of the spinal alignment were the trunk imbalance, trunk inclination, lateral deviation, and surface rotation. Results: The muscle thickness of the EOA muscle increased more significantly in the right side than the left side (p<0.05). Each left and right difference in the muscle thickness between the male and female group showed a significant difference (p<0.05) except for the TrA thickness. Significant positive correlations were observed between the ES and lateral deviation and between the TrA with trunk imbalance. Conclusion: These results suggest that asymptomatic men have a greater trunk muscle thickness than women but there was no difference between the left and right in healthy adults. The trunk muscle thickness of ES, TrA is related by the spinal alignment.