Abnormal tapetum degradation and anther development in cytoplasmic male sterility SaNa-1A are the main reasons for the anther abortion. SaNa-1A is a novel cytoplasmic male sterility (CMS) line of Brassica napus derived from somatic hybrids of B. napus-Sinapis alba, and SaNa-1B is the corresponding maintainer line. Ultrastructural comparison between developing anthers of sterile and maintainer lines revealed abnormal subcellular structure of pollen mother cells (PMCs) in the CMS line. The PMC volume and size of nucleus and nucleolus in the CMS line were smaller than those in the maintainer line. The abnormal tapetum cell development and delayed tapetum degradation inhibited microspore development. Finally, anther abortion in the CMS line occurred. Physiological and biochemical analyses of developing anthers and mitochondria revealed that over-accumulation of reactive oxygen species (ROS) in the SaNa-1A and deficiency in antioxidant enzyme system aggravated the oxidization of membrane lipids, resulting in malondialdehyde (MDA) accumulation in anthers. High MDA content in the CMS line was toxic to the cells. ROS accumulation in SaNa-1A also affected anther development. Abnormal structure and function of terminal oxidase, which participates in the electron transport chain of mitochondrial membrane, were observed and affected the activity of cytochrome c oxidase and F₁F₀-ATPase, which inhibited ATP biosynthesis. Proline deficiency in SaNa-1A also affected anther development. Few hybridization signals of programmed cell death (PCD) in tetrads of SaNa-1A were identified using TdT-mediated dUTP Nick-End Labeling assay. PCD was not obvious in tapetum cells of SaNa-1A until the unicellular stage. These results validated the cytological differences mentioned above, and proved that abnormal tapetum degradation and anther development in SaNa-1A were the main reasons for the anther abortion.