The Djoumouna periurban forest, in Brazzaville, was studied according to three plots of one hectare each. With an original area of 8.5 ha in the 1990s, this forest has declined by 49%, following human activities, in twenty years. The floristic inventory revealed 898 trees of dbh ≥ 10 cm, either 299 ± 18.68 trees.ha-1 on average; Specific diversity reveals 106 species corresponding to 36 families, the most important of which, following IVF are: Fabaceae (78%); Rubiaceae (41.2%); Euphorbiaceae (35.2%). According to the IVI, the dominant species are: Pentaclethra eetveldeana (22%); Elaeis guineensis (15%); Pentaclethra macrophylla (14%); Plagiostyles africana (13.5%); Hymenocardia ulmoides (10.5%). The Shannon index averages is 3.44 ± 0.10 for maximum diversity average of 4.14 ± 0.03. The Pielou index average is 0.83 ± 0.02, no dominance is found within the phytocenoses. Data from biological indices coupled with floristics show that this ecosystem has a high specific diversity, against a low floristic richness. The diametric structure coupled with the skewness reveal a bad natural regeneration of the ligneous trees. The similarity coefficients of Jaccard (30-47%) and Sørensen (46-64%) show a difference floristic composition. This heterogeneity is the appearance of the area-species curve. The analysis of diaspores shows the importance of sarcochores (73% of species and 62% of individuals), hence zoochoria as the main mode of dissemination. This mesophilic and tropophilous forest is characterized by the Fabaceae of which Pentaclethra eetveldeana and Pentaclethra macrophylla are the markers.