To investigate the role of JNK1 in metabolism, male ob/ob and diet-induce obese mice were treated with a JNK1-specific antisense oligonucleotide (ASO) or control ASO at 25 mg/kg or with saline twice a week for 6 and 7 weeks, respectively. JNK1 ASO reduced JNK1 mRNA and activity by 65-95% in liver and fat tissues in both models. As compared to controls, treatment with JNK1 ASO did not change food intake, but lowered BW, fat pad weight and whole body fat content. The treatment increased metabolic rate. In addition, the treatment markedly reduced plasma cholesterol levels, and improved liver steatosis and insulin sensitivity. These positive observations were accompanied by the following changes: 1) increased mRNA levels of AR-₃ and UCP1 by > 60% in BAT; 2) reduced mRNA levels of ACC1, ACC2, FAS, SCD1, DGAT1 and DGAT2 by 30-60% in WAT; and 3) reduced mRNA levels of ACC1, FAS, G6Pase and PKC by 40-70%, and increased levels of UCP2 and PPAR by > 2-fold in liver. JNK1 ASO treated mice demonstrated reduced levels of pIRS1^Ser302 and pIRS1^Ser307 and increased levels of pAkt-^Ser473 in liver and fat in response to insulin. JNK1 ASO-transfected mouse hepatocytes showed decreased rates of de novo sterol and fatty acid synthesis and an increased rate of fatty acid oxidation. These results indicate that inhibition of JNK1 expression in major peripheral tissues can improve adiposity via increasing fuel combustion and decreasing lipogenesis, and could therefore provide clinical benefit for the treatment of obesity and related metabolic abnormalities.