The effect of antioxidants on photochemical reaction of pefloxacin by UVB (290~320 nm) was investigated and the possible mechanism of phototoxicity on the skin was also studied. The photo-degradation of pefloxacin by UVB was suppressed by cysteine, reduced glutathione and ascorbic acid, but was promoted by ${alpha}-tocopherol$. Squalene, accounts for more than 10% of skin surface lipids, was peroxidized by pefloxacin through both radical and singlet oxygen mechanism.