Objective To study the therapeutic effects of Alisma orientale extract (AOE) on nonalcoholic fatty liver disease in rats and systematically determine the underlying mechanism. Methods Collected samples of an oleic acid?palmitic acid mixture were applied to adipohepatic HepG2 cells after AOE treatment. ER stress marker proteins GRP78, CHOP, and XBP?1, as well as JNK1, p?JNK1, and STAT3 were detected by Western blotting. Serum SOD, AST, ALT, TG, HDL, and LDL levels were measured after establishment of high fat diet?induced nonalcoholic fatty liver disease and AOE treatment in SD rats as confirmed by oil red staining. CYP2E1 and CYP2A5 gene expression levels were determined in liver tissue by RT?PCR. Results After AOE treatment, JNK1, p?JNK1, GRP78, CHOP, and XBP?1 expression showed obvious dose?dependent downregulation, while STAT3 expression was increased. Serum AST, ALT, TG, and LDL levels were reduced dose?dependently, but SOD and HDL levels were increased. CYP2E1 and CYP2A5 gene expression levels were decreased obviously in liver tissue, especially in the high dose group. Conclusions AOE inhibited ER stress in adipohepatic HepG2 cells, and JNK signaling pathway and metabolic enzymes in vivo to treat nonalcoholic fatty liver disease.