Phytoplanktons are important oil-producing materials in lake basins. As a case study, lacustrine petroleum source rocks in the Jiyang depression of the Bohai Bay basin are studied. They are mainly distributed in the Shahejie Formation (Eocene-Oligocene) and can be divided into three types; calcareous laminated shales, calcareous laminated mudstones and organic-rich laminated shales. Optical, SEM and BSEI analyses of these source rocks reveal that there are algal (coccoliths and dinoflagellates) laminae in the rocks. An individual algal lamina is generally composed of one species, such as Reticulofenestra bohaiensis in coccolith laminae, and Fromea as well as Bohaidina in dinoflagellate laminae. This suggests that algal laminae be formed from algal blooms in the ancient lake. Other components of source rocks included organic matter laminae, carbonate laminae and clay-rich laminae. Besides, it is suggested in the paper that the sedimentation of organic matter laminae and carbonate laminae be also closely related to algal blooms. We thus conclude that the laminated lacustrine source rocks in the Jiyang depression were formed from algal blooms, which is the primary mechanism in the formation of lacustrine petroleum source rocks.