We experienced a case of dissociative stupor with decorticated posture in a 71-year-old woman after neck clipping of the brain aneurysm. Decorticated posture is observed with severe midbrain disorder caused by brain herniation. In this case, therefore, severe brain stem damage was suspected, although light reflex was observed and respiration was stable. In addition, in this case, the aneurysm was located at the junction of the internal carotid artery and the posterior communicating artery. Therefore, surgical damage to midbrain was not likely to have happened. Postoperative computed tomography showed no abnormality in this region. Two days later, her symptom disappeared, and she could clearly recollect memory of these two days just after surgery. However, she could not follow directions because of a sense of fear. Thus her symptom might be based on a kind of mental disorder. Patients with conversion disorder react abnormally to stimulus or stress. Stupor was diagnosed as due to defect of reaction to stimulus of sound, light and touch. She was diagnosed as dissociative stupor of the conversion disorder type. In this case, decorticated posture was caused by mental disorder. We have to pay attention not only to brain tissue damage but also to psychological damage of patients.