Polyphenols in methanolic extracts from Sonneratia caseolaris seeds were determined by HPLC coupled with electrospray mass spectrometry and quantified by HPLC coupled with ultraviolet/visible detection in order to evaluate their stability of the extracts during 6 months of storage. Antioxidant activity was measured by using TEAC assay, and the free radical scavenging activity was monitored in the initial 3 months of the stability evaluation. Study on the effect of temperature on the stability of the methanolic extract indicated that gallic acid, luteolin-7-O-glucoside, and luteolin were unstable at 25 and 45°C during storage time. At -80 and 4°C the quantities of luteolin-7-O-glucoside reduced while those of luteolin increased at the same time. The results suggested that luteolin-7-O-glucoside was hydrolyzed to luteolin. Therefore the polyphenols should be kept at low temperature. Antioxidant activity of methanolic extract was preserved in all temperatures during the first 3 months.