Peroxisome proliferator-activated receptor alpha (PPARα) ligands have been reported to suppress cancer growth. However, the role of PPARα in hepatocarcinogenesis remains unclear. We investigated the functional significance of PPARα in HCC. PPARα-knockout (PPARα-/-) mice were more susceptible to diethylnitrosamine (DEN)-induced HCC at 6 months compared with wild-type (WT) littermates (80% versus 43%, P < 0.05). In resected HCCs, TUNEL-positive apoptotic cells were significantly less in PPARα-/- mice than in WT mice (P < 0.01), commensurate with a reduction in cleaved caspase-3 and caspase-7 protein expression. Ki-67 staining showed increased cell proliferation in PPARα-/- mice (P < 0.01), with concomitant up-regulation of cyclin-D1 and down-regulation of p15. Moreover, ectopic expression of PPARα in HCC cells significantly suppressed cell proliferation and induced apoptosis. The anti-tumorigenic function of PPARα was mediated via NF-κB as evidenced by inhibition of NF-κB promoter activity, diminution of phosphor-p65, phosphor-p50 and BCL2 levels, and enhancing IkBα protein. Chromatin immunoprecipitation analysis confirmed PPARαdirectly binds to the IkBα promoter. In conclusion, PPARα deficiency enhances susceptibility to DEN-initiated HCC. PPARα suppresses tumor cell growth by inhibiting cell proliferation and inducing cell apoptosis via direct targeting IκBα and NF-κB signaling pathway.