Abstract: The paleoenviroment of Dongying Sag is studied by focusing on the features of Ostracoda fossil community and paleoecology, especially the dominance(dm) and diversity(Hs)of Ostracoda. The five enviroments in the late shahejie Ⅲ(Oligocene) of Dongying Sag are distinguished: 1) floodplain-coastal lake; 2) alluvial delta plain; 3) shallow lake-delta fron; 4)sub-deep lake-prodelda ;5)lake bay.