Abstract: To investigate the causes of lack of fruit set in Chinese chestnut, a cultivar named ‘Yanshanzaofeng’ was employed in Qianxi County, Hebei province, and the process of sexual reproduction was examined by microscope. The results showed that ①catkins with floral primordia were formed in the buds of one-year olds shoots in later April. The sex differentiation was completed in early May. Later, the anther wall layers developed completely by mid-May and consisted of epidermis, endothecium, middle layers and glandular tapetum, and the anther wall was of the basic type. Microspore mother cells underwent meiosis through simultaneous cytokinesis in later May and gave rise to tetrads of microspores, which were tetrahedrally arranged. Mature pollens contained two cells with three germ pores. Anthers were dehiscent and pollen grains shed between early June and middle June. ② The female flowers were formed in early-May, and can be divided into female flower cluster primordium differentiation stage, alabastrum primordium differentiation stage, stigma primordium differentiation stage, stigma elongation stage, ovary formation stage and blooming stage. The ovule primordium was formed in middle June. The ovary has eight or nine locules, and each locule contained two anatropous ovules which were bitegmic and crassinucellate. The embryo sac was mature between later June and early July, and the development of embryo sac conformed to the Polygonum type. ③ When the anthers were dehiscent, the pollen tube passed through the basal style about two weeks. The pollen tube came into the embryo sac between later June and early July, and the syngamy was of premitotic type. The development of the endosperm was nuclear endosperm. The zygote developed from the proembryo to the clavate, globular, torpedo-shaped and cotyledon stage embryoid, and the development of embryo was of Onagrad type. ④The mature embryo formed in early August and the endosperm disappeared between later August and early September. The 71.7% abortive ovules were observed in the development of female gametophytes in middle July which showed the cavity of embryo and degeneration of nucellus tissue. The abortive ovules were identified as one of the major factors causing the low seed set in ‘Yanshanzaofeng’. Thus, It was the key to prevent the abortive ovules for the chestnut production.