A hot water extract of herbal tea from the seed with hard shell of Trapa japonica Flerov. showed moderate antioxidative activity against the rat brain homogenate model. The antioxidative compounds in the hard shell were isolated by chromatographic methods, and identified for the first time as gallic acid ( 1 ), 1,2,3-trigalloyl-β-glucoside ( 2 ), 1,6-digalloyl-β-glucoside ( 3 ), 1,2,6-trigalloyl-β-glucoside ( 4 ), eugeniin ( 5 ), and 1, 2, 3, 6 tetragalloyl-β-glucoside ( 6 ) by using HR-MS and NMR analyses. Their respective antioxidative activities (IC₅₀) were 14 μM ( 1 ), 5.2 μM ( 2 ), 8.8 μM ( 3 ), 0.25 μM ( 4 ), 0.26 μM ( 5 ), and 0.27 μM ( 6 ). The antioxidative compound in the herbal tea was also clarified as gallic acid ( 1 ) by an HPLC analysis.