OBJECTIVE： To study chemical compounds of Monoraphidium dybowskii， and to investigate the in vitro antibacterial 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 physicochemical properties and NMR. MIC of 4 isolated compounds to Pseudomonas aeruginosa， Candida albicans， Bacillus subtilis and Escherichia coli were determined by resazurin disc test. Free radical scavenging rate （concluated by IC50） and reducing capacity were measured 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 stigmasterol， 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.