OBJECTIVE:To study chemical compounds of Monoraphidium dybowskii,and to investigate the in vitro antibacte-rial and antioxidant activities of isolated compounds. METHODS:The ethanol extract of M. dybowskii were extracted with aether petrolei,ethyl acetate and n-butyl alcohol. The ethyl acetate extract was separated from M. dybowskii and chemical components were analyzed by sillica gel column chromatogram,HPLC and GC-MS. Their structures were identified according to physicochemi-cal properties and NMR. MIC of 4 isolated compounds to Pseudomonas aeruginosa,Candida albicans,Bacillus subtilis and Esche-richia coli were determined by resazurin disc test. Free radical scavenging rate(concluated by IC50)and reducing capacity were mea-sured by 1,1-diphenyl-2-picryl-diazanyl (DPPH) and ferric reducing antioxidant power (FRAP) assay. RESULTS:Compounds 1-6 were obtained from E4 and E5 segments of ethyl acetate extract of M. dybowskii,and their structures were identified as stigmas-terol,diisonoyladipate,indole-3-carboxylic acid,(+)-epiloliolide,(-)-loliolide,5-hydroxy-3,4-dimethy-5-pentyl-2(5H)-furanone. MIC of compounds 3-6 were 10-500 μg/mL,and IC50 ranged 22.02-71.01 μg/mL;FRAP ranged (62.04 ± 5.36)-(281.22 ± 8.3) μmol/L. CONCLUSIONS:M. dybowskii contains multiple lipid and alkanoic acid,and possesses certain in vitro antibacterial and antioxidant activities.