Fragmentation of the Atlantic forest is extensive and a major reason for biodiversity loss in the South American continent. Today only a fraction of the pre-Colombian forest cover is still left and predominantly represented by isolated fragments. In the highlands of southern Brazil the biome constitutes the eco-region Araucaria moist forest that has a unique plant composition and many endemic species. This study has focused on one small fragment of Araucaria moist forest within the state Rio Grande do Sul in southern Brazil. The study was performed as an inventory of the tree species present in the fragment to acknowledge its diversity and tree species composition. From this the origin and conservation status for the fragment could be concluded. The inventory demonstrated high species diversity for the fragment with a Shannon index of nearly 3,43. Also many similarities to natural Araucaria moist forest were evident with the occurrence of 12 of the 19 native species most represented in natural forests. A high percentage of dead trees were documented with nearly 6,5 % . Further the endemic species Dicksonia sellowiana and Nectandra megapotamica were recorded within the fragment, D. sellowiana is an endangered species in Brazil. These results indicated a fragment with natural origin as part of the mosaic composition in the eco-region rather than a result of human land use such as plantation or regrowth of agricultural land. Thus the fragment may be a remnant from the great Araucaria forest expansion occurring in the second part of the Late Holocene with an age of up to 1100 years. This study suggests that the surveyed Araucaria moist forest fragment has significant conservation status and deserves accurate conservation measures to be taken for the continuing existence of this unique and threatened eco-region.