Abstract: Abundant ostracod fossils have been found in the Mesozoic and Cenozoic rocks which are most well developed throughout China.This paper is a preliminary study of their sedimentary facies with emphasis laid on ostracods zonation and their ecological aspects. Ostracod fossils are classified into three types,continental,transitional and marine.From study of living environment and morphology of the fauna and with reference to the lithology of rocks,it has been established that in China the continental type is most widespread,and the other two are of limited distribution.Ostracod fossils are also used as index fossils for zonation or subdivision of rocks,particularly the oil bearing strata.8 fossil assemblages for the coastal region of Bohai in Eocene and Oligocene,12 for Daqing Oil Field in Cretaceous,11 for Jiangsu from late Cretaceous to Quaternary and 7 for Zaidam Basin in Quaternary. Ostracod assemblages of the Bohal coastal region and the Daqing Oil Field are dealt with at greater length in respect to their morphological,ecological and biogeographic aspects etc.Of particular interest,a great part of the 8 fossil assemblages of upper Eocene-Oligocene in the Bohai region are of new genera and species of varying sizes and ornamentation, totalling as much as twenty odd genera and several hundred species.The 12 assemblages of Cretaceous in Daqing include ten odd genera and more than one hundred species. Mention is also made of some marine and brackish organisms like foraminifers etc.