The radiation effect on a hydrophobic room temperature ionic liquid (RTIL), 1 butyl methyl imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C4mim][NTf2]), was studied by 7 irradiation under nitrogen atmosphere. Accompanied by color darkening and increase of light absorbance in a wide wavelength range, a distinct absorption peak at around 290nm for irradiated [C4mim][NTf2] appeared when acetonitrile was used as solvent, and the intensity of the peak enhanced with increasing dose. The spectrophotometric study on the irradiated RTILs containing 1,3 dialkylimidazolium cations associated with different inorganic anions revealed that the peak is ascribed to the radiolysis products of the [C4mim]+. Arid the wavelength of the peak was affected by alkyl chain length on imidazolium cation, while the intensity of the peak was influenced by anions. With incorporating a little amounts of oxidants, such as KMnO4 and HNO3 into irradiated [C4mim][NTf2], the intensity of the peak at 290nm decreased obviously and the decoloration of [C4mim][NTf2] occurred, suggesting that the peak at 290nm is assigned to the colored species and the species can be oxidized.