Congenital basilar invagination (BI) is a craniocervical deformity marked by odontoid prolapse into the skull base. The foramen magnum angle (FMA), which is formed by Chamberlain's line and McRae's line has not been fully studied. The study aimed to investigate the FMA and its relationship with other craniocervical parameters.Participants were divided into controls, type A BI and type B BI. Parameters included Chamberlain line violation (CLV), atlantodental interval, clivus height, clivus anteroposterior dimension, FMA, basal angle, clivo-axial angle, head and neck flexion angle, Boogard's angle, and subaxial cervical spine lordosis angle. Comparision of these parameters among the three groups and correlation analysis between FMA and other parameters was performed. The significance level was set at P<0.05.A total of 111 controls, 111 type A BI patients, and 62 type B BI patients were enrolled. The FMAs in the control, type A BI, and type B BI groups were 6.21 (3.67,8.71), 22.16±6.61, and 22.39 (17.27, 31.08) °, respectively. Correlation analysis revealed correlations between the FMA and other variables. In the two BI subgroups, FMA was significantly correlated with CLV, clivus height, clivus anteroposterior dimension, basal angle, clivo-axial angle, and Boogard's angle.The FMA in patients with BI was approximately 22° and approximately 6° in controls, indicating that the FM in BI had a greater tilt. As a pathological condition, FMA can reflect the degree of BI. Clivus hypogenesis is a reason for excessive tilt of the FM.Copyright © 2022. Published by Elsevier Inc.