Abstract: The antioxidant stability of polyphenols extract from Porphyra dentate was evaluated by measuring the scavenging capacity against hydroxyl free radicals under various conditions of temperature, light, microwave and pH. The results indicated that polyphenols from Porphyra dentate have strong scavenging activity on hydroxyl free radicals, the scavenging activity was increased with the increase of polyphenol concentration. The scavenging activities of polyphenols from Porphyra dentate was stronger below 60℃. The scavenging activities of polyphenols from Porphyra dentate decreased with the increase of temperature and heating time when temperature was over 60℃. The illumination decreased antioxidant activities followed as:light>avoid light, and the scavenging activity was decreased more with illumination time prolonging. Scavenging rate of hydroxyl radical was sharply reduced at 10min of microwave, and then scavenging rate was slowly decreased with the microwave treatment time extending. Scavenging activity of polyphenols on hydroxyl free radical was relatively stable at pH3.0⁷.0. In the processing and storage process, polyphenols extract from Porphyra dentate should be avoided high temperature and light, and paid attention to the effect of microwave and pH at the same time.