Aim: To provide basic osteometric data of the following diameters anteroposterior, transverse, right and left oblique, and shape of human foramen magnum in Indian skulls and CT images. A total of 100 adult human skulls from the Department of Anatomy and 100 CT Brain images taken in the Department of Radiology, Mahatma Gandhi Medical College and RI, Pondicherry were evaluated. Methodology: Maximum transverse, anteroposterior, right and left oblique diameters of foramen magnum were calculated using sliding vernier calipers to an accuracy of 0.1mm and visually assessed for foramen magnum shape classification into- oval, round, tetragonal, hexagonal and irregular. The same parameters were also evaluated in adult CT Brain images after 3D reconstruction. Results: The mean anteroposterior, transverse, right oblique and left oblique diameters in dry skulls and CT images were 3.39cm, 2.87cm, 2.90cm, 2.92cm and 3.49cm, 2.98cm, 3.04cm, 3.04cm respectively. The dimensions in CT images were significantly higher than dry skull and significantly higher in CT images of males compared to females. Commonest shape noted was oval followed by irregular and the least was round in both dry skull and CT images. Conclusion: The foramen magnum plays an important role as a landmark because of its close relationship to key structures such as the brain stem and the spinal cord. It is of particular interest in field of forensic medicine to identify fire victims and also used for intracranial surgical approaches. Size of foramen magnum has an etiological significance in herniaton of cerebellar tonsil. With such clinical significance there is paucity of literature regarding its variations in size and shape in context to different races. Hence the present study.