Several serine/threonine kinases reportedly phosphorylate serine residues of IRS-1 and thereby induce insulin resistance. In this study, to investigate the effect of mTOR/raptor on insulin signaling and metabolism in K/KAy mice with genetic obesity-associated insulin resistance, a dominant negative raptor, COOH-terminally deleted raptor (raptor-ΔC_T), was overexpressed in the liver via injection of its adenovirus into the circulation. Hepatic raptor-ΔC_T expression levels were 1.5- to 4-fold that of endogenously expressed raptor. Glucose tolerance in raptor-ΔC_T-overexpressing mice improved significantly compared with that of LacZ-overexpressing mice. Insulin-induced activation of p70S6 kinase (p70^S6k) was significantly suppressed in the livers of raptor-ΔC_T overexpressing mice. In addition, insulin-induced IRS-1, Ser³⁰⁷, and Ser^636/639 phosphorylations were significantly suppressed in the raptor-ΔC_T-overexpressing liver, whereas tyrosine phosphorylation of IRS-1 was increased. PI 3-kinase activation in response to insulin stimulation was increased approximately twofold, and Akt phosphorylation was clearly enhanced under both basal and insulin-stimulated conditions in the livers of raptor-ΔC_T mice. Thus, our data indicate that suppression of the mTOR/p70^S6k pathway leads to improved glucose tolerance in K/KAy mice. These observations may contribute to the development of novel antidiabetic agents.