To evaluate the effects of sodium arsenite on the activity, the mRNA and the protein expression of CAT in human keratinocyte cell line (HaCaT).The activity of catalase (CAT) was detected by ultraviolet direct velocity assay. RT-PCR was used to detect the mRNA expression of CAT and Western blotting was conducted to detect the protein expression of CAT.If the cells were treated with higher than 5.0 micromol/L sodium arsenite, the activity, mRNA and protein expression of CAT were decreased significantly and in a dosage dependent fashion (P < 0.05).CAT is inhibited by sodium arsenite in the transcription, translation and activity levels.