Abstract:This paper reports the relationship between the structures of different-staged leaf cells of Stevia rebaudiana Bertoni and distribution of stevioside content.The electron microscopy observation shows that mature leaf cell, during its floral initiation period, contains a huge vacuole with a lot of inelusions which appear as different-sized granules or vesicles.Furthermore, the suhcellular separation of mature leaf of S. rebaudiana is performed with differential centrifugation, and the stevioside in various parts is respectively extracted out and microanalyzed. The results show that most of stevioside is in 12000 Xg supernatant (this part mainly contains vacuolar inclusions and soluble cytoplasm). Relating these results with the result of cytochemical study, we think that soluble cytoplasm may be the main site of the synthesis of UDPG, which is critical in the synthesis of stevioside, and stevioside is stored in vacuoles.The analysis of stevioside for the different-staged leaf cells shows that its cntent of stevioside in floral initiation period is the highest. Considering the relationship between the content of stevioside and the leaf-age, we may conclude that the floral initiation period is exactly the suitablest reaping time.