A 23-year-old female presented with dural sinus thrombosis caused by protein S deficiency. She suffered superior sagittal sinus thrombosis 6 days after delivering her first child. Past history showed deep vein thrombosis at the age of 20. While conservative management was initiated because of the potential risk of increasing intracranial hemorrhage, several hours later she deteriorated rapidly because of severe brain swelling with massive hemorrhage. The patient died despite surgical decompression. Autopsy disclosed organized thrombus in the superior sagittal and transverse sinuses. Although the total concentration of protein S was normal, the free protein S concentration and protein S activity were decreased. Protein S deficiency is a rare cause of dural sinus thrombosis, but is associated with a high mortality rate, so accurate diagnosis and urgent intervention are required.