AbstractSome dietary fibers originated from insects such as silkworm (Sericin) and others along with constituents of several representative seaweeds such as wakame Undaria pinnatifida; hijiki Hizikia fusifome; and kombu Laminaria japonica, were found to have fairly large reaction rates determined by quenching experiments of emission spectra in the near‐infrared region λ:_{max}1270 nm for singlet oxygen 1O2, Cypridina luminescence method for superoxide, and peroxide value (POV) for autoxidation. The determined reaction rates are between 103 ‐ 105 (g/L){‐1} s{‐1} for the insect and the plant dietary fibers; the larger ones are as large as that of ascorbic acid, 1.93 × 104 (g/L){‐1} s{‐1} for singlet oxygen. Most of these seaweed constituents also showed antioxidative activity against autoxidation and superoxide as well as their immunological enhancing activity. These results suggest a possibility that dietary fibers that are supposed to prevent the large‐intestine cancer by their physical properties may prevent the cancer, at least in parts, by their chemical, antioxidative activity.