We identified two splicing variants of the human arsenic (III oxidation state) methyltransferase (AS3MT) gene in HepG2 cells. One splicing variant was an exon-3 skipping (Δ3) form which produced a premature stop codon, and the other was an exon-4 and -5 skipping (Δ4,5) form which produced a 31.1 kDa AS3MT protein, lack of methyltransferase activity. In addition, we found that exposure of HepG2 cells to hydrogen peroxide (H2O2) resulted in increased levels of a novel spliced form skipping exon-3 to exon-10 (Δ3– 10). These data suggest that abnormal alternative splicing of AS3MT mRNA may affect arsenic methylation ability and the splicing of AS3MT pre-mRNA was disconcerted by oxidative stress.