Introduction Severe obesity often present with nonalcoholic fatty liver disease (NAFLD) and obstructive sleep apnea (OSA). Emerging researches suggest OSA plays an important role in NAFLD development and progression while the relationship between OSA and NAFLD is still conflicting . The interaction of OSA and NAFLD should be further evaluated as obesity surges. The purpose of this study was to assess the prevalence of OSA and NAFLD in patients with obesity undergoing bariatric surgery, and evaluate the association between OSA and severity of NAFLD. Methods 141 patients with severe obesity undergoing preoperative polysomnography and intraoperative liver biopsy during bariatric surgery was investigated. Clinical, anthropometric variables, liver enzymes, fasting blood glucose, fasting serum insulin, and homeostasis model assessment (HOMA-IR) were measured. The severity of NAFLD was assessed by degree of steatosis, ballooning, intralobular inflammation and NAFLD activity score (NAS). The diagnosis and severity assessment of OSA was based on an apnea/hypopnea index (AHI). Results OSA was diagnosed in 127 (90.07%), NAFLD in 124 (87.94%), and non-alcoholic steatohepatitis (NASH) in 72 (51.06%) patients. There was a statistically difference in body mass index (BMI), waist circumstance, neck circumstance, high-density lipoprotein-cholesterol (HDL), fasting insulin, and HOMA-IR among the three groups divided by the severity of AHI. In addition, the distribution of hepatic steatosis grades among the three groups was statistically different (P=0.025). AHI was significantly associated with HOMA-IR and hepatic steatosis when assessing the association between OSA parameters and liver histology in NAFLD(P

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