We compared the anti-diabetic effects of mitochondrial uncoupler Niclosamide Ethanolamine (NEN) and the first-line drug metformin, and examined potential combinatory effect or drug drug interaction between the two compounds in mouse models. We treated the High-Fat Diet (HFD)-induced diabetic and the db/db mice with NEN (~200 mg/kg.day), metformin (~75 mg/kg.day), or the two compounds together through oral administration, and monitored the exposure of the two compounds and effect on improving diabetic conditions. At the dosages tested in the present study, the co-treatment leads to plasma and liver exposures of NEN and metformin that are similar to treatment with each drug alone. In both animal models, NEN alone is more effective than metformin alone in reducing blood glucose. The co-treatment is significantly better than metformin alone in lowering blood glucose. NEN treatment alone is effective in reducing hepatic lipid accumulation, while metformin treatment does not. Moreover, the co-treatment appears to have a better effect in reducing hepatic steatosis and liver damage than NEN treatment alone. Our data support that NEN and metformin co-treatment does not compromise the hypoglycemic effect of metformin, and the co-treatment may have additional benefit of improving hepatic steatosis.