Objective To investigate the correlation between thoracic kyphosis (TK) and vertebral rotation in patients with Chiari malformation and scoliosis. Methods Thirty patients with Chiari malformation and scoliosis and 34 patients with adolescent idiopathic scoliosis (AIS) who underwent EOS imaging from Jul. 2017 to Jul. 2019 in Nanjing Drum Tower Hospital of Nanjing University Medical School were retrospectively selected. According to the Cobb angle and apical vertebral segment of the coronal main thoracic curve, patients with TK ≤ 20° in the AIS group and TK ≥ 30° in the Chiari group were enrolled. The EOS imaging of the patients was selected and three-dimensional reconstruction was performed. The Cobb angle, apical vertebral segment, vertebral rotation angle and TK of the coronal main thoracic curve were measured by the threedimensional reconstruction image, and the mean vertebral rotation (MTR) of the main thoracic curve was calculated. Pearson correlation analysis was used to study the correlation between TK and MTR in the main curvature region in the two groups. Results A total of 24 patients (12 pairs) were enrolled, including four males and eight females in the AIS group, and five males and seven females in the Chiari group. There was no differences in the gender composition between the two groups (χ²=0.00, P>0.05). The mean age of the two groups were 12-18 (15.25±2.00) years and 11-18 (14.42±2.43) years (P>0.05). The mean Cobb angle of the coronal main thoracic curve of the two groups were (58.13±11.45)° and (55.35±12.35)° (P> 0.05). The mean TK and MTR (absolute value) of the AIS group and the Chiari group were (13.89±6.35)° vs (37.11±9.40)° (t=-6.38, P<0.01) and (24.62±2.78)° vs (21.53±1.66)° (t=3.94, P=0.002), both with significant differences. Pearson correlation analysis showed that TK was associated with MTR in the both groups (r=-0.667 and -0.645, P=0.018 and 0.024). Conclusion Under the same coronal plane, the larger TK in patients with Chiari malformation and scoliosis may be caused by vertebral rotation, which suggests that spine surgeons should pay attention to sagittal rotation as well as coronal correction in Chiari malformation patients with scoliosis.