Abstract: Objective: To simulate the ischemia of neurosurgery and study the duration of brain ischemia in rats by preconditioning therapy of mild hypothermia.Methods: Reversible middle cerebral artery occlusion (MCAO) was induced by employing a intraluminal suture.SD rats were assigned randomly to one of six groups (n=12) as follows: 1hr，2hrs，3hrs MCAO-normothermic group and 1h，2h，3h MCAO-mild hypothermia group.Neurological examinations，infarcted volume.Results: In 1 h and 2h MCAO groups，applying mild hypothermia before the ischemia can improve the neurological outcome and reduced the infarcted volume.In 3h MCAO-mild hypothermia groups，there is no effect of brain protection.Conclusion: Preconditioning therapy of mild hypothermia can reduce the injury of brain ischamia，the duration of brain ischemia is with in 2hrs.