The fruits of Sonneratia caseolaris (L.) Engl. were extracted successively with n-hexane, chloroform, ethyl acetate and methanol using a soxhlet process to give four corresponding extracts. The antimicrobial activity of these extracts was evaluated by Broth microdilution assay against 8 microbial strains. The results showed that the chloroform extract exhibited the antibacterial activity against B. subtillis with the MIC value 400 µg/mL, both the chloroform and ethyl acetate extracts possessed the antifungal activity against A. niger with the same MIC value. The phytochemical study on the chloroform extract led to the isolation of four triterpenoids including oleanolic acid (1), 3β-O-acetyloleanolic acid (2), maslinic acid (3), and arjunolic acid (4), in which (4) is previously undescribed compound from such species. The structures of these compounds were elucidated by ESI-MS, NMR spectral analysis and compared with published data.