Nonmarine fossil dinoflagellates with marine origin have been found widely  from the Mesozoic-Cenozoic terrigenous, especially salty lacustrine sediments in China. Because of the characteristic of dinoflagellates in favorable oil-generating, they are considered as an important hydrocarbon resource of nonmarine Cretaceous-Tertiary in China. The Eocene Lower Ganchaigou Formation is the uppermost hydrocarbon source rock in Qaidam basin. Abundant 4-methylsteranes and dinosteranes, which are generally thought to be biomarkers of dinoflagellates, were preserved in the hydrocarbon source rocks and crude oils. However, no reliable fossil dinoflagellates have been discovered in the area heretofore. This paper reports the new finding of fossil dinoflagellates from the Eocene Lower Ganchaigou Formation in Qiadam basin. A monotype but comparatively abundant dinoflagellate assemblage was recovered from the Formation of Well Kun-2 in the north margin of Qiadam basin. The assemblage mainly consists of abundant cysts of Subtilisphaera, and shows obviously nonmarine characteristics. All these provide direct evidence for dinoflagellates as an important source of Tertiary hydrocarbon in Qiadam basin. The studies indicated the Middle Eocene in which dinoflagellates were discovered might be is the most favorable hydrocarbon source rocks in Qaidam basin, and the sedimentary environment was a salty lake in wet and hot climate. 