Abstract: Dinoflagellate cyst in the Dayao Bay, southern Liaoning, where the scallops and oysters were largely culti-vated, was investigated from 2006 to July 2007. 10 species of dinoflagellate cysts in total were identified, which repre-sented 4 general kinds of dinoflagellate cysts, including 2 species of Protoperidinioid, 5 species of Gonyaulacoid and 1 species of Calciodinellid. One species of Gymnodinioid, Ensiculifera Balech and one species of Fragilidium mexicanum of dinoflagellate cysts were freshly recorded in the sea area of China. Gonyaulacoid was the most common and abundant cyst type of all species of dinoflagellate cysts, which took up 50%, Protoperidinioid was 30%, Calciodinellid and Gymnodinioid were each 10% respectively. One species of Protoperidinioid dinoflagellate cysts could not be identified to genus. Three species of dinoflagellate cysts belonged to heterotrophic dinoflagellate cysts, and seven belonged to autotrophic dinoflagellate cysts. Two species of cyst produced by toxic dinoflagellate (Alexandrium tamarense, A. catenella and Gymnodinium catenatum) were also recorded, which produced Paralytic Shellfish Poisoning. The seasonal change curve of dinoflagellate cysts density had two obviously fastigium, which occurred on January and August 2007. The first highest density of dinoflagellate cysts appeared in January 2007, 1090 cysts/g W, and the second highest ap-peared in August 2007, 855 cysts/g W. The lowest density of dinoflagellate cysts appeared in October 2007, only 280 cysts/g W. The average density of dinoflagellate cysts was 695.5 cysts/g W all the year. The value of Shannon-Weaver diversity index (H) varied between 0.17 to 0.55, which were similar on each month except on September and October 2007. The average of species diversity index was 0.46.