Objective: The major objective was to explore the protection machanism of hyperbaric oxygen treatment on liver cells of GK (Goto Kakizaki) rats with type 2 diabetes mellitus. Methods: GK rats (n=24) were divided randomly and evenly into model group (received gastric irrigation with purified water 5 mL/kg/d once per day), metformin hydrochloride group (MH, n=8, received gastric irrigation with metformin hydrochloride 250 mg/kg/d once daily), and hyperbaric oxygen group (HBO, n=8, inhaled pure oxygen under a constant pressure 0.15 Mpa for 30 min per day, and received gastric irrigation with purified water 5 mL/kg/d once per day), while healthy Wistar rats were used as normal control group (N, n=8, received gastric irrigation with purified water 5 mL/kg/d once per day). After 3 weeks of treatment, rats were fasted for 12 hours, measured body weight and fasting blood glucose. And then the rats were anesthetized with isoflurane, and immediately separated the liver, liver tissue were taken electron microscopic examination and biopsy paraffin. Some paraffin sections were stained using PAS and Masson, some samples were stained using the immunohistochemistry approach with Caspase-3 anti-bodies, the apoptosis of liver cells was examined using the TUNEL technique. Results: These resultant data showed that the pathological changes of the HBO group was less than those of the model group with respect to hepatic steatosis, fibrosis, glycogen increases, increased apoptosis and mitochondrial swelling, degeneration. The index of cell apoptosis and Caspase-3 expression in the HBO group showed significant difference (P<0.05) with that in the model control, and no significant difference (P>0.05) compared with that in the normal control and metformin hydrochloride group. Conclusion: This implies that the HBO treatment might protect liver tissues, improve the structure of the liver by easing increased apoptosis induced by hyperglycemia in diabetic rat liver.