The Chang- 7 organic- rich shale in the Upper Triassic Yanchang Formation of Ordos Basin, with high abundance of organic matter and good type, is the main source rock of Mesozoic oil system, which has great hydrocarbon generation potential. The enrichment of organic matter in this unit mainly depends on the high paleoproductivity and the sufficient supply of organic matter during the Chang- 7 sedimentary period. Many researchers have accepted the occurrence of algal bloom in the Chang- 7 period, but there are still no direct and powerful evidences to prove it. In this study, algal fossils characterized by single species and high abundance are found in carbonate nodules and the vicinity of vein pyrite, which provides a good evidence for the algal bloom and strong support for the nutrition of hydrothermal activities during the Chang- 7 period. Methods: Microscope and scanning electron microscopy (SEM) are used to observed algae fossils. Energy spectrum analyzer is used to analyze the element composition of algae fossils. Thin section observation is carried out at the State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum (Beijing). SEM observation and energy spectrum analysis are carried out the Microstructure Laboratory for Energy Materials, China University of Petroleum (Beijing).Results:Many algal fossils have been found on the edge of vein pyrite. They are single species, densely distributed and rich in organic matter.Conclusion: In the vicinity of vein pyrite and carbonate nodules in Chang- 7 organic- rich shale, algal fossils have been well preserved due to pyritization, calcitization and dolomitization. These algal fossils are characterized by single genus and species, and high distribution abundance. The discovery of algae fossils has important geological significance. It can not only provide direct and powerful evidence for algae bloom during the Chang- 7 sedimentary period, but also provide support for the nutritional role of hydrothermal activities in the same period.