The c-Jun N-terminal kinase (JNK) pathway is known to be activated under diabetic conditions and to possibly be involved in the progression of insulin resistance. In this study, we examined the effects of modulation of the JNK pathway in liver on insulin resistance and glucose tolerance. Overexpression of dominant-negative type JNK in the liver of obese diabetic mice dramatically improved insulin resistance and markedly decreased blood glucose levels. Conversely, expression of wild type JNK in the liver of normal mice decreased insulin sensitivity. The phosphorylation state of crucial molecules for insulin signaling was altered upon modification of the JNK pathway. Furthermore, suppression of the JNK pathway resulted in a dramatic decrease in the expression levels of the key gluconeogenic enzymes, and endogenous hepatic glucose production was also greatly reduced. Similar effects were observed in high fat, high sucrose diet-induced diabetic mice. Taken together, these findings suggest that suppression of the JNK pathway in liver exerts greatly beneficial effects on insulin resistance status and glucose tolerance in both genetic and dietary models of diabetes.