Pollen-ovule ratio (P/O) of the angiosperms, the ratio of pollen grains to ovules, can reflect more indications than floral morphology for the breeding systems. This article reviews numerous literatures about P/O to explore its relationship with breeding system, pollination ecology, evolution, and taxonomic, etc. Because of the special pollination mutualism ecology and the minute pollen size of Ficus, anther/ovule ratio (A/O) substitutes for P/O to explore their pollination ecology and phylogeny.