Tumor necrosis factor (TNF)-α inhibits insulin action; however, the precise mechanisms are unknown. It was reported that TNF-α could increase mitochondrial reactive oxygen species (ROS) production, and apoptosis signal–regulating kinase 1 (ASK1) was reported to be required for TNF-α–induced apoptosis. Here, we examined roles of mitochondrial ROS and ASK1 in TNF-α–induced impaired insulin signaling in cultured human hepatoma (Huh7) cells. Using reduced MitoTracker Red probe, we confirmed that TNF-α increased mitochondrial ROS production, which was suppressed by overexpression of either uncoupling protein-1 (UCP)-1 or manganese superoxide dismutase (MnSOD). TNF-α significantly activated ASK1, increased serine phosphorylation of insulin receptor substrate (IRS)-1, and decreased insulin-stimulated tyrosine phosphorylation of IRS-1 and serine phosphorylation of Akt, and all of these effects were inhibited by overexpression of either UCP-1 or MnSOD. Similar to TNF-α, overexpression of wild-type ASK1 increased serine phosphorylation of IRS-1 and decreased insulin-stimulated tyrosine phosphorylation of IRS-1, whereas overexpression of dominant-negative ASK1 ameliorated these TNF-α–induced events. In addition, TNF-α activated c-jun NH₂-terminal kinases (JNKs), and this observation was partially inhibited by overexpression of UCP-1, MnSOD, or dominant-negative ASK1. These results suggest that TNF-α increases mitochondrial ROS and activates ASK1 in Huh7 cells and that these TNF-α–induced phenomena contribute, at least in part, to impaired insulin signaling.