Cytological observation on microsporogenesis of inter-subspecific semi-sterility hybrid F₁ and its parents was conducted by using technique of paraffin section and extrusion chromosomes on slides.The results indicated that the anther wall of hybrid F₁ and its parents was normal.However,the meiosis of some microspore mother cells in hybrid F₁ was abnormal,e.g.chromosome pairing unequally,aberrant dyad and tetrad formation.The hybrid F₁ forms univalent at metaphase Ⅰ,delayed chromosomes and chromosomal bridge could be found at anaphase Ⅰ,delayed chromosome also could be found at anaphase Ⅱ.In summary,the main causes of the pollen semi-sterility of hybrid F₁ might be caused by the aberrant chromosome behavior of some microspore mother cells in hybrid F₁ at meiosis metaphase and anaphase.