Objective To investigate the optimal mild hypothermia course and cerebral temperature of the neonatal rats after hypoxic-ischemic brain injury. Methods The posthypoxic-ischemic rats of experimental group (n=60) were placed in the glass jars immeresd in water bath held constaut at either 29 C or 3l C for 24h, 48h or 72h. While the rats of room temperature group (n=22) were stayed in room air. Blood glucose, blood gases and neuropathology findings were studied to determine the therapeutic effects. Results The brain temperature droped 3C or 5C when enviro ment temperature was 31C or 29C respectively. The blood glucose remained normal. Neuropathology findings reveled that the brain damage of experimental rats reduced 46%～86% compared to the room temperature group. Conclusion Reducing the cerebral temperature by 4～5 C for 72 hours after hypoxic-ischemic brain injury can lead to superior protective effect.