ABSTRACT The Genus Casearia is a rich source of cleordane - type diterpenes. To search for practical sources of potential cleordane - type diterpenes, and to study the structure and activity relationship of cleordane - type diterpenes, the twigs and leaves of Casearia membranacea Hance ( Flacourtiaceae ) was collected for phytochemical and anti-tumor investigation. Bioassay-directed fractionation of an ethyl acetate layer of Casearia membranacea has resulted in the isolation of six new clerodane-type diterpenes. The structures of these cleordane-type diterpenes were established and designated as Caseamembrins G¡ãL¡]1¡ã6¡^and their derivatives compounds 7 and 8. The structures of compounds 1¡ã8 were determined by application of NMR techniques included 1H NMR, 13C NMR, DEPT, COSY, HMQC, HMBC, NOESY and another physical methods which include MS, UV, IR and optical rotation, and the published reports about the data of related compounds. The spectral data of 1¡ã8 are in conformity with the basic skeleton of cleordane-type diterpenes previously isolated from Casearia membranacea Hance. The basic structures of 1¡ã8 contain two 6- menbered ring. The structures were identified as rel-(2S,5R,6R,8S,9S,10R, 18S,19R)-2-hydroxy-6-butanoyloxy-18,19-acetyloxy-18,19-epoxy-cleroda-3,13(16),14-triene (Caseamembrin G, 1¡^¡Brel-(2S,5R,6R,8S,9S,10R,18S, 19R)-2-hydroxy-6,18-dibutanoyloxy-19-acetyloxy-18,19-epoxy-cleroda -3,13(16),14-triene (Caseamembrin H, 2¡^¡Brel-(2S,5R,6R,8S,9S,10R,18S ,19R)-2-(2-methylbutanoyloxy)-6-hydroxy-18-methoxy-19-acetyloxy-18,19-epoxy-cleroda-3,13(16),14-triene¡]Caseamembrin I, 3¡^¡Brel-(2R,5R ,6R,8S,9S,10R)-2-(2-methylbutanoyloxy)-6-hydroxy-cleroda-3,13(16),14 -triene-18,19-dicarboxaldehyde ( Caseamembrin J, 4¡^¡Brel-(2S,5R,6R,7R, 8S,9S,10R)-2,7-diacetyloxy-6-hydroxy-cleroda-3,13(16),14-triene-18,19-dicarboxaldehyde¡]Caseamembrin K, 5¡^¡Brel-(2S,5R,6S,7R,8S,9S,10R, 18S,19R)-2-butanoyloxy-6,7-dihydroxy-18-butanoyloxy-19-acetyloxy-18,19-epoxy-cleroda-3,13(16),14-triene¡]Caseamembrin L, 6¡^¡A¤Îrel-(2S,5R ,6R,8S,9S,10R,18S,19R)-2-O-acetyl-6-butanoyloxy-18,19-acetyloxy-18,19-epoxy-cleroda-3,13(16),14-triene ( 7¡^¡Brel-(2S,5R,6R,8S,9S,10R,18S,19 R)-2-O-acetyl-6,18-dibutanoyloxy-19-acetyloxy-18,19-epoxy-cleroda-3,13(16),14-triene¡]8¡^. It is worthy to mention that this is the first report of the isolation of 1¡Ð6 from a natural source and their derivatives compounds 7 and 8. Compound 1 exhibited moderate cytotoxicity against¡]Hepa59T/VGH¡^ ,¡]KB¡^and ¡]Hela¡^cancer cells , but compound 3 and 4¡B5 show no activity against those cancer cell lines.