In this study, we found that chronic high-fat diet may enhance the severity of NAFLD by enhancement of oxidative stress and activating ER stress (GRP78, PERK, eIF2α, CHOP, JNK, and the ratio of p-IRE1/IRE1) as well as the proapoptosis pathway (Bcl-2, which is an antiapoptotic protein, Bax, and Caspase-3) and apoptosis (TUNEL positive staining) in the liver. The gene discussed is ERN1; the disease is metabolic dysfunction-associated steatotic liver disease.