Abstract: The development of ovule was studied after self- and cross- pollination of Citrus grandis cv.Guanximiyou (GXM) and Duweimiyou (DWM) by paraffin section.GXM after selfing showed two ovular developmental patterns (Type Iand TypeⅡ).In Type I,nucellar cells were deformed 5 days after pollination (ADP),followed by the atrophy of embryo sac,but outer integument continued to develop for 50 ADP;in TypeⅡ,endosperm developed abnormally and eventually aborted.DWM after selfing showed endospermless.Nucellar cells and embryo sac simultaneously atrophied 35 ADP.The development of ovule in crosses between GXM and DWM was normal and result in normal seed formation.These results demonstrate that GXM and DWM are self-incompatible,which is the main reason for seedless fruit.The recognition response and inhibition process occur in different development stage,before fertilization in Type Iand after fertilization in Type Ⅱ,respectively.In contrast,self-cross pollen tubes in DWMare unable to reach embryo sac with self-incompatible reaction occurred only before fertilization.It appears that aborted seed sizes are related to the time of self-incompatible reaction.In addition,our results will facilitate investigation on the genetic basis of self-incompatibility in Citrus and provide solutions to juice sac granulation,dehiscent fruit and cracked valve often associated with seedless cultivars.