Compositional changes of hydrotreated naphthenic oil during ultraviolet (UV) radiation were studied in detail. First, liquid-solid chromatography was used to separate the initial oil and the UV-irradiated oil into three different fractions: saturates, aromatics, and polars. Then, each fraction's compositional changes were evaluated using a series of instrument technologies including gas chromatography-mass spectroscopy (GC-MS), infrared spectroscopy (IR), atmospheric pressure chemical ionization mass spectroscopy (APCI/MS), and X-ray photo electron spectroscopy (XPS), respectively. The results show that after UV radiation, the UV-irradiated sample exhibited a decrease in saturates concomitant with an increase in the aromatic and polar fractions. For saturates and aromatics, the UV-irradiated sample exhibited a decrease in alkanes, diaromatics, and polyaromatics concomitant with an increase in other aromatics. The UV light had no significant effect on the cycloalkanes. For polars, a relatively large amount of oxygen-containing compounds, such as hydroxyl, carbonyl, and carboxyl groups, was formed in the presence of UV light. XPS data show that a type of S 2pa compound in the initial oil disappears and could be converted to SO 4 2− . No obvious changes of molecular weight distribution were observed. A part of polar compounds in the UV-irradiated oil might come from aromatic photooxidation in the initial oil.