Several conservation programs have been started for the critically endangered Sichou oak (Quercus sichourensis) since 2007. These programs include detailed field investigations, seedling cultivation and research on the fruit biology of the species. In this study, we first report on the five mature individual trees found in our 9-year field investigation. Thus far, a total of 10 mature individuals have been recorded. All Q.sichourensis trees are healthy and most produce healthy acorns. Acorns of Q.sichourensis are large with dry masses of 8.0-14.0g. These acorns had high moisture contents at collection and died shortly after (7-28d) when dried with silica gel. Characteristics of Q.sichourensis acorns varied between populations. Compared with the acorns from Funing, the acorns collected from Ceheng were bigger, more viable (germination percentage was up to 96%), less sensitive to desiccation, and germinated faster. Q. sichourensis occurs in regions with a distinct 5-6 month dry season. Habitat degradation is largely responsible for the rareness of Quercus sichorensis, but desiccation sensitivity of the acorns may also limit the regeneration of the species and potentially lead to its continued rareness. As a species with extremely small populations (PSESP), Q.sichourensis is facing high risk of extinction and should be defined as a Critically Endangered species in the global IUCN Red List.