Abstract: Karlodinium veneficum is a kind of toxic dinoflagellate and has broken out in China in recent years.This paper studied the effects of Karlodinium veneficum to Brachionus plicatilis, Artemia salina, Neomysis awatschensis and Calanus sinicus which belong to different sizes and trophic levels.We found that it has greater effects to Brachionus plicatilis and Artemia salina than Neomysis awatschensis,and it almost has no effects to Calanus sinicus.The LC50(medium lethal concentration)of Brachionus plicatilis is about 3400 cells/mL in 48 h.The LC50 of Artemia salina is about 4800 cells/mL in 96 h.The LC50 of Neomysis awatschensis is about 89000 cells/mL in 96 h and the survival rate of Calanus sinicus is about 80% under the circumstances.The effects of different fractions of Karlodinium veneficum are different on experimental organisms.The cell lysate has greatest effects on Brachionus plicatilis and Neomysis awatschensis.The extracellular fluid has greatest effects on Artemia salina.The Brachionus plicatilis, Artemia salina,and Calanus sinicus ingest Karlodinium veneficum and as the density increased,the feeding rate increase in 24 h.So we can concluded that Karlodinium veneficum has different effects to marine organisms and different toxic mechanisms and pathways.When the algal density is 104cells/mL(the algal blooms have existed),the Brachionus plicatilis and Artemia salina have been greatly influenced.It may has greater effects to Plankton ecosystem if algal blooms break outand can know the hazard more comprehensive by studying the effects of harmful algal to different sizes and trophic levels organisms.