The recently created Lomami National Park has an extensive Buffer Zone where local communities are allowed to use forest products sustainably. However, the management of this Buffer Zone is hampered by inadequate understanding of the floristic composition and the impacts of harvesting certain plant products. To fill in this data gap, we established and sampled 30 vegetation plots of 40 × 40 m, and investigated the population structure of 11 useful tree species preferred by local communities. We found that the Buffer Zone is mostly composed of mixed-species undisturbed old growth forest. It contains nine tree species of international conservation concern (listed in the IUCN Red list and/or apart from the Red list, Gibourtia demeusei, not been assessed by IUCN Red list, is listed in CITES Appendix II since January 2017; UNEP-WCMC, Review of selected Dalbergia species and Guibourtia demeusei, UNEP-WCMC, 2017). The floristic diversity of the Buffer Zone, which requires further investigation as some species remained unidentified. Most preferred tree species (including Garcinia kola and Milicia excelsa) are abundant and showed a reverse-J size distribution, indicating a relatively stable population structure. For these species, current levels of exploitation seem sustainable. Further research is needed for two nonabundant species of conservation concern (Autranella congolensis and Michelsonia microphylla). Although timber/firewood commercial harvesting is currently limited in the study area, the management plan of the Buffer Zone should consider these risks given increasing commercial hunting. Surveys and permanent plots provide essential information to guide the management of newly formed protected areas.