Objective To study the effect of cumulus cell apoptosis on the structure of human oocyte cultured in vitro and to probe into the reason why cumulus cell apoptosis rate can be used clinically to predict the developmental potential of oocyte. Methods Cumulusenclosed oocyte complexes (COCs) at the GV stage were cultured in vitro. The apoptosis rate of cumulus cells for every COC was evaluated with HE staining, DAPI staining and TUNEL labeling methods. Oocytes were divided into two groups according to their cumulus cell apoptosis rate. Structure of oocytes was observed under light microscope and transmission electron microscope. Mature oocytes at MⅡ stage were fertilized and cultured in vitro for two days. Then the embryos were evaluated according to their morphology under light microscope.Results The oocytes with low cumulus cell apoptosis rate had a normal structure and a promising developmental potential, while the oocytes with high cumulus cell apoptosis rate had deformed structures and a poor developmental potential. Some deformed oocytes had uneven perivitelline space. For oocytes with a small perivitelline space, the organelle development was slow compared with those with a large perivitelline space. Some had unevenly piled organelles. Secondary lysosomes and large space probably caused by the decopmounding of lysosomes were found in the ooplasm of these oocytes. The deformed oocytes appeared to have lots of swollen mitochondria with blurred cristae and membrane. Some of these oocytes had fractured first polar bodies and abnormally thick or thin zona pellucida. The cell junctions and microvilli of cumulus cells reduced as well. Conclusion The effect of cumulus cell apoptosis on the structure of oocyte cultured in vitro was revealed. The reaso