Objective To analyze and classify the characteristic of plain X-ray, CT scan, MRI and pathologic findings in ossification of the ligamentum flavum of the thoracic spine. Providing the clinic criterion to verify the extent and progression of ossification of ligaments based on medical imaging findings. Methods Twenty-four patients with thoracic myelopathy due to ossification of the ligamentum flavum underwent surgical decompression posteriorly. There were 18 males and 6 females with age ranging from 42-76 years (mean, 57.9 years). The morphology of ossification on the CT scan was divided into isotype and non-isotype. The signal intensity of ossification was compared with that of spinal cord on T2 stage in MRI, whose results were divided into four types, such as no signal, low signal, iso signal and high signal. There were two types of ossification in pathologic findings, the mature and immature ossification. 73 segments of ossified ligaments removed from surgery were evaluated and classified individually on the base of their X-ray, CT scan, MRI and pathologic findings. 27 segments of ossified ligaments were examined immunohistochemically by use of TGF-?1 antibody. The relationship between classification of X-ray, CT scan, MRI and pathology of the ossified ligaments were compared on the basis of individual segment, to determine whether there was correlation between these findings. Results The relationship between the pathologic findings of the 73 ossified fragments and the manifestation for the CT classification and MRI signal showed: 18 isotype ossification on CT scan turn out to be mature; and among the 55 non-isotype ossification, 51 were immature and 4 were mature. 22 no signal ossifications on MRI were confirmed as mature type; all the 50 low signal ossifications were immature type; and 1 iso signal ossification was immature type. 100% were matched between MRI and pathological findings, 94.5% matched between CT and pathological findings. Conclusion The extent and progression of ossification of ligaments may be verified and predicted clinically on the base of CT scan and MRI findings, which provide the clinic criterion to guide the extent and timing of decompression.