Surface modification on a polyimide film by pulsed ultraviolet (UV) laser irradiation with a fluence below its ablation threshold was studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and UV-vis spectroscopy. It was observed that a photochemical reaction occurred and hydrophilic groups, such as -OH and -COOH, formed on the polyimide surface after irradiation. In addition, a ripple microstructure formed on the surface when the angle of incidence of the laser beam was 20-50 degrees. The contact angle of the polyimide surface with water decreased and the adsorption ability of the surface to a water-soluble dye clearly increased after laser irradiation.