Abstract: Objective: To evaluate the manifestations of Posterior shoulder Instability on CT, MRI. Methods: 16 cases of Posterior shoulder Instability were examined both by CT, MRI and arthroscopy and to analyze the manifestations on CT, MRI. Results: Posterior shoulder Instability included Glenoid hypoplasial, Reverse osseous Bankart fracture, Reverse Hill-Sachs fracture, Bennett lesion, Lesions of the posterior labrum and capsular complex, posterior subluxation or dislocation of the humeral head relative to the glenoid fossa. Conclusion: Lesions of the glenoid and humerus, posterior glenoid labrum, and posterior joint capsule can be identified on CT, MRI in patients with posterior shoulder instability. CT, MRI is a good examine method of Posterior shoulder Instability.