Abstract： Objective:To investigate the characteristics of magnetic resonance imaging changes and the curative effects of early different pressure hyperbaric oxygen (HBO) on moderate and severe traumatic brain injury in rats.Methods:Sixty healthy clean male adult Sprague-Dawle rats were randomly divided into four groups by random number table: traumatic brain injury group (TBI group), sham operation group (sham group. Only opening the skull, but no head strike), 0.15 MPa HBO group (giving 0.15 MPa HBO to TBI rats), and 0.25 MPa HBO group (giving 0.25 MPa HBO to TBI rats). Each group has 15 rats. All rats were subjected to modified neurological severity score (mNSS) evaluation and MRI examination on day 1 (D1), day 3 (D3), and day 7 (D7) respectively. Three regions of interest (ROI) were taken from the high-signal regions of the magnetic resonance DWI image. The apparent diffusion coefficient (ADC) of each region was measured. The ratios of the ADC of the lesion side to the ADC of the normal contralateral side, i. e. the relative ADC (rADC), were compared.Results:(1) On D1, all the rADC values of ROIs in each group were not statistically significant ( P>0.05). On D3, the rADC values of different ROIs in 0.15 MPa HBO and 0.25 MPa HBO groups were all lower than those in TBI group ( P< 0.05), but all higher than sham group ( P<0.05); there was no significant difference between 0.15 MPa HBO and 0.25 MPa HBO groups ( P>0.05). On D7, the rADC values of different ROIs in 0.15 MPa HBO and 0.25 MPa HBO groups were all lower than those of TBI group ( P< 0.05); there was no significant difference when those compared with sham group ( P>0.05), and there was no significant difference between 0.15 MPa HBO and 0.25 MPa HBO groups ( P>0.05). (2) On D1, there was no significant difference in mNSS scores among TBI group, 0.15 MPa HBO group and 0.25 MPa HBO group ( P>0.05), but the mNSS scores of TBI group, 0.15 MPa HBO group and 0.25 MPa HBO group were all significantly higher than that of sham group ( P<0.05). On D3, the mNSS scores of 0.15 MPa HBO and 0.25 MPa HBO groups were all lower than that of TBI group ( P<0.05), but all higher than that of sham group ( P<0.05); there was no significant difference between 0.15 MPa HBO group and 0.25 MPa HBO group ( P>0.05). On D7, the scores of mNSS in 0.15 MPa HBO and 0.25 MPa HBO groups were all lower than that in TBI group ( P<0.05), but all slightly higher than that in sham group ( P>0.05). Conclusion:There is definite curative effect of early HBO on moderate and severe TBI, but the pressure of HBO may not be the key factor in determining prognosis.