Abstract: Alkenones are ubiquitous in ocean sediments and have been widely used in paleoceanography and paleoclimatology. Alkenones can be used to reconstruct the CO₂ concentration of seawater in geological history, which is key to understanding the mechanism of climate evolution and predicting future climate change. In modern open ocean environments, the main producers of alkenones are Noelaerhabdaceae, including the calcifying haptophytes (coccolithophores) Emiliania huxleyi and Gephyrocapsa spp. It is generally believed that the most reliable producers of alkenones in the Cenozoic marine sediments belong to genus Reticulofenestra of Family Noelaerhabdaceae. By comparing the absolute abundance of coccoliths and alkenones in the Early Miocene sediments at IODP Site U1501 in the South China Sea, it is found that the abundance of Cyclicargolithus spp. coccoliths and alkenones are well correlated (r=0.44, p＜0.01). Therefore, we believe that Cyclicargolithus spp. is the main producer of alkenones in the Early Miocene, followed by Reticulofenestra spp. In addition, we measured the coccolith length of each species and estimate the diameter of coccolithophores cells. We inferred that the high contribution of Cyclicargolithus spp. to alkenones is due to its larger cell diameter. Our finding is helpful to evaluate the feasibility of using carbon isotope fractionation to reconstruct the atmospheric CO₂ concentration in geological history.