Objective: To study the effects of mild hypothermia on cerebral oxygen partial pressure, carbon dioxide partial pressure, pH and body temperature (PbrO2, PbrCO2, pHbr and BT) in patients with acute severe head injury.Methods: Thirty-eight patients with acute severe head injury were treated with mild hypothermia, meantime PbrO2, PbrCO2, pHbr and BT were monitored in order to study the changes of PbrO2, PbrCO2, pHbr and BT.Results: In patients with acute head injury, mild hypothermia obviously increased PbrO2, decreased PbrCO2 and CO2 accumulation and acidosis in brain tissue. BT was 1℃-1.5℃ higher than rectal temperature(RT) after injury. The BT and RT were decreased when the patients were treated with mild hypothermia, but at the same time the difference between BT and RT was increased.Conclusions: In patients with acute severe head injury the direct monitoring of PbrO2, PbrCO2, pHbr and BT was safe and reliable, and is helpful in estimating prognosis and mild hypothermia therapy.