Objective To assess the protective effect of the normobaric oxygen (NBO)and hyperbaric oxygen (HBO)on the acute ischemic stroke of rat using MR diffusion weighted imaging (DWI).Methods 30 adult male SD rats were subjected to right middle cerebral artery occlusion (MCAO)model using the suture method and transferred into a MRI scanner at 30 minutes after the onset of MCAO,then 30 rats were randomly divided into 3 groups (n=10 in each group),control,NBO and HBO group.NBO group rats were exposed to 100% oxygen for 2 hours,HBO group rats were administrated with hyperbaric oxygen for 2 hours respectively at 45 minutes after the onset of MCAO.Brain MRI scanner was performed at 12 h after the onset of MCAO,then pathological change of brain tissue was observed with hematoxylin-eosin (HE)staining and compared with DWI-infarct lesion.The relative cerebral infarction area and relative apparent diffusion coefficient (rADC)values of every group were measured on ADC maps.Results High signal intensity on MRI was found in the right cerebral ischemic region on DWI in three groups.The increased rates of the infraction lesion area in both NBO and HBO group were lower than that in control group (P<0.01),however,there was no significant difference between NBO and HBO group (P>0.05).The reduction rates of rADC in NBO and HBO group were significantly less than that in the control group (P<0.01),however,there was no significant difference between NBO and HBO group (P>0.05).HE staining confirmed that the cerebral infraction occured in three groups.There were significant positive correlations of the relative infarction lesion area between HE staining and DWI at 12 h after MCAO in three groups (P<0.01).Conclusion MRI shows that NBO and HBO could reduce the growth rates of cerebral infarction lesion area and the reduction rates of rADC,which proves that NBO and HBO have the neuroprotective effect on acute ischemic stroke of rats.