An infamous poison, arsenic also has been used as a drug for nearly 2400 years; in recently years, arsenic has been effective in the treatment of acute promyelocytic leukemia. Increasing evidence suggests that opposite effects of arsenic trioxide (ATO) on tumors depend on its concentrations. For this reason, the mechanisms of action of the drug should be elucidated, and it should be used therapeutically only with extreme caution. Previously, we demonstrated the opposing effects of ERK1/2 and JNK on p21(WAF1/CIP1) (p21) expression in response to ATO in A431 cells. In addition, JNK phosphorylates c-Jun (Ser(63/73)) to recruit TGIF/HDAC1 to suppress p21 gene expression. Presently, we demonstrated that a high concentration of ATO sustains ERK1/2 phosphorylation, and increases c-Fos biosynthesis and stability, which enhances p21 gene expression. Using site-directed mutagenesis, a DNA affinity precipitation assay, and functional assays, we demonstrated that phosphorylation of the C-terminus of c-Fos (Thr(232), Thr(325), Thr(331), and Ser(374)) plays an important role in its binding to the p21 promoter, and in conjunction with N-terminus phosphorylation of c-Fos (Ser(70)) to transactivate p21 promoter expression. In conclusion, a high concentration of ATO can sustain ERK1/2 activation to enhance c-Fos expression, then dimerize with dephosphorylated c-Jun (Ser(63/73)) and recruit p300/CBP to the Sp1 sites (-84/-64) to activate p21 gene expression in A431 cells.