We investigated the quenching activity of capsaicin （CAP） against singlet oxygen （¹O₂） using nano second time-resolved laser flash photolysis. As a result, the ¹O₂ quenching rate constants （k_Q） of CAP was 7.7×10⁵ M^－1sec^－1, which was found to be much less active than that of α-tocopherol （1.9×10⁸ M^－1sec^－1） and 2, 5-diphenyl-3, 4-benzofuran （2.8×10⁹ M^－1sec^－1）. However, CAP quenches with ¹O₂ more rapidly than the natural decay rate of ¹O₂ （8.3×10⁴ sec^－1） in ethanol. Next, we measured the k_Q values, the guaiacol derivatives, which make up the main skeleton of CAP, and discussed the ¹O₂ - quenching active site of CAP. Consequently, N-benzylacetamide, a related compound which does not have the phenol moiety of CAP but does have acetamide moiety, did not show ¹O₂ quenching activity. This result indicates that the phenolic hydrogen of CAP is the site of ¹O₂ quenching activity. Furthermore, it was also clarified that the ¹O₂ quenching activity of CAP is affected by the type of side chain with para position of the hydroxyl group of CAP.