2-(3-Benzoylphenyl)propanoic acid (ketoprofen), one of the nonsteroidal anti-inflammatory drugs, causes photocontact dermatitis by ultraviolet (UV) light as a side effect. In this study, we examined radical reactions induced by ketoprofen in the lipid membranes under UV irradiation using egg yolk phosphatidylcholine (egg-PC) liposomal membranes containing 5- or 16-doxyl stearic acid (5- or 16-DSA), which carry nitroxyl radical at the 5- or 16-position of the fatty acid chain, respectively. When the suspension of liposomal membrane was mixed with ketoprofen and irradiated with UV, electron spin resonance signal of 5- and 16-DSA in the membrane decreased. The decay consisted of fast decay and subsequent slow decay. The overall decay for 5-DSA was faster than that for 16-DSA. The rate of slower decay of 16-DSA increased with ketoprofen concentration. The bulk lipid in the membrane affected the rate of slower decay of 5-DSA; the rate increased with the amount of egg-PC and decreased in the rigid membrane composed of dipalmitoylphosphatidylcholine. When spin trapping studies with α-(4-pyridyl 1-oxide)-N-tert-butylnitrone (POBN) and 5,5-dimetyl-1-pyrroline-N-oxide (DMPO) were performed in ketoprofen solution, C-centered radical adducts of POBN and superoxide anion radical adducts of DMPO were detected after UV irradiation. POBN suppressed the signal decay of 5-DSA in the liposomal membrane, whereas superoxide dismutase accelerated it. These results support that ketoprofen penetrates the lipid membrane and induces a radical reaction near the polar region in the membrane, and that ketoprofen-related C-centered radical is involved in the radical reaction.