The present invention relates to the Bacteroides eggerthii KGMB05336 strain and a use thereof, wherein, through the analysis of the microbiome in patients with non-alcoholic fatty liver disease that is caused by obesity, the Bacteroides eggerthii strain was identified as a significant strain. Oral administration of the Bacteroides eggerthii strain to animal models of obesity and non-alcoholic fatty liver disease caused by obesity resulted in body weight reduction, liver weight reduction, decrease of fat accumulation in visceral and liver tissues, and reduction in serum levels of AST (U/L), ALT (U/L), cholesterol (mg/dL), and glucose (mg/dL), thus demonstrating that the Bacteroides eggerthii strain can serve as a novel means for the prevention, improvement, or treatment of obesity and diseases caused by obesity, such as fatty liver, hepatomegaly, non-alcoholic fatty liver disease, and diabetes.