The flora of the Patte d’Oie forest of Brazzaville is studied according to three plots of 0.5 ha, corresponding each to one of the islet, currently constituting it. With an original area of 240 ha, put in reserve in 1938, the natural forest of the Patte d’Oie of Brazzaville covers 95 ha, less than 39% compared to initial area. Because the other main forests of Brazzaville have disappeared without delivering their biodiversity, this study is the first one which emphasizes the flora of this single ecosystem of Congo. The total floristic inventory revealed 459 trees of dbh≥10 cm on 1.5 ha; it is an average of 306 trees?ha?1. 31 species are listed for 17 families, and the best represented of which (Fabaceae, Euphorbiaceae and Rubiaceae) have a diversity varying from 13% to 19% of the inventory and a tree number oscillating from 15% to 36%. The Shannon (H') biological diversity index of the three plots is of 2.6 on average, for a maximum diversity (H'max) average of 2.9 versus an average true diversity of 15. The Pielou index is on average 0.9, whereas that of Simpson is 0.9. The coefficients of similarity of Jaccard (41.4% to 57.1%) and of the coefficient of S?rensen (58.5% to 72.7%) show the variations in the floristic composition. The followed ecological parameters show that this ecosystem is very few diversified compared to the tropical wet dense forests on the one hand, and, on the other hand, a non constant floristic composition is not. The analysis of the diametric structure by plot and the dominant and/or characteristic species give erratic curves, evidence of an insufficiency of the regenerative potential. Indeed, approximately 70% of the species are characteristic of the disturbed ecosystems (Macaranga sp.,Caloncoba welwitschii,Hymenocardia ulmoides,and Anthocleista sp.).