Biological evaluations of bicyclo[6.4.0]dodecenone derivatives on antimalarial activity in vitro against Plasmodium falciparum and cytotoxicity against human KB cells were made. (+/-)-(1R*,4S*,7R*,8S*)-4-tert-Butyl-dimethylsiloxy-5,5-dimethlyl-1-methyl-9-methylene-7-phenylbicyclo[6.4.0]dodec-2,11-dien-10-10-one (15) exhibited potent antimalarial activity, whereas (+/-)-(1R*,7R*,8S*)-1-methyl-9-methylene-7-phenylsulfonybicyclo-[6.4.0]dodec-2,11-dien-10-one (14) showed significant cytotoxic activity in human KB cells. Both 14 and 15 possess, as a structural character, the exo-methylene moiety in their 6-membered ring of the 8-6 fused ring system.