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A tannin, EA (1), and other nine non-tannins compounds, gallic acid (2), quercetin (3), myricetin (4), 3-O-methylellagic acid-4′-O-α-L-rhamnopyranoside (5), quercetin-3-O-β-D-galactopyranoside (6), kaempferol-3-O-β-D-glucoside (7), quercetin-3-O-β-D-glucuronide (8), quercetin-3-O-rutinoside (9), 3,3′,4-tri-O-methylellagic acid-4′-O-β-D-glucopyranosyl (10) were isolated from a valuable medicinal plant, Eucalyptus citriodora. Structural identification of these compounds was conducted using 1H NMR and 13C NMR spectroscopy and comparing their spectral data with those previously reported in literatures. The anti-inflammatory effects of EA were evaluated in ethanol-induced acute gastric ulcer mice models in our study. The result demonstrated that the intragastric administration of EA significantly prevented the gastric ulceration caused by ethanol treatments. Especially, the gastric tissue in the middle-dose EA (100 mg/kg) showed few ulcerations with only slight focal congestion which indicated that it has a significant protective effect on gastric ulcer by increasing the IL-10 and PGE2 levels, and reducing the IL-6, TNF-α, GAS and COX-2 levels. In addition, the middle-dose EA has no adverse effect on liver and kidney. These findings imply that EA exerts gastroprotective effects by means of its anti-inflammatory effects and may be a potential drug for anti-ulcer treatment.