Abstract: Stamen fusion in plants is defined and classified in this paper. Stamen fusion includes both coales-cence and cohesion of any part of stamens or the fusion of whole stamens into one or more units. The structure formed by stamen fusion can be further fused with style and a gynostemium is always formed, such as in Aris-tolochiaceae and Stylidiaceae. Three main types of stamen fusion are identified: filament fusion, anther fusion, and fusion with both filament and anther. Stamen fusion can be found in about 70 families of seed plants and the complex distributions of these types in the different families indicate they had evolved for several times inde-pendently in angiosperms. The filament fusion is always found in primitive angiosperms with polypetalous flower, while the anther fusion and fusion with both filament and anther are usually correlated with advanced groups with sympetalous flower (long and narrow floral tube is always presented), which indicates that the different type of stamen fusion may be of adaptive significance with the floral syndrome to promote the plant’s fitness through either female or male avenues (pollen receipt or pollen dispersal). The structures and functions of the various types of stamen fusion received little, if any, attentions in experimental studies at present. Theoretically, filament fusion can fix the stamens at the relatively stable positions and make the stamens steady to support the moving pollina-tors and protect the ovary. Anther fusion can assemble the anthers to the same position and facilitate all the anthers touching the same part of pollinator’s body, which can greatly enhance the precision of outcrossing pollination. Fusion with both filament and anther can greatly change the morphology of stamens and level of herkogamy besides the effects caused by filament fusion and anther fusion. In conclusion, the morphology and position of stamens can be changed when the stamens are fused, and the level of herkogamy, interference between female and male organs can be altered greatly, consequently the mating patterns in plant population can be affected. However, the adaptive significances of different types of stamen fusion are still in need of further experimental studies to illuminate the possible functional differences of varied types of stamen fusion. The types of fusions and degrees of fusions could be used as an important character for taxonomic classifications since the structure and morphology of stamen are relatively stable.