The study intended to determine the floristic composition of an ecotonal forest zone between the Gilbertiodendron dewevrei forest and the mixed forest of Mbiye Island Forest Reserve. Data were collected from a 50 m x 100 m plot set in each investigated biotope (ecotonal forest (interface), Gilbertiodendron dewevrei Forest and mixed forest). These diversity indices were computed using PAST and BIODIV R 1.0. A total of 736 specimens grouped into 153 species and 71 families were investigated. Coelocaryon botryoides Verm. was represented in all the three investigated biotopes, and it was the most abundant species in the ecotonal forest.While, Gilbertiodendron dewevrei (De Wild) J. Leonard was the most abundant species in the remaining two biotopes. The most dominant family in the ecotonal forest was the Myristicaceae, while the Fabaceae family dominated in the other two biotopes. The terrier area was estimated at 28.2 m2/ha, 22 m2/ha and 21.8 m/ha in Gilbertiodendron dewevrei Forest, mixed forest and the ecotonal forest, respectively. A strong similarity was detected between the two first biotopes (communities) (0.57) compared to the mixed forest (0.43).