Cytotoxic effects of an aqueous extract of a plant used in traditional Congolese medicine Klainedoxa gabonensis Pierre ex Engl. (KG) were studied on the aggressive U87 glioblastoma cell line. An aqueous extract (E2) was prepared from barks of KG and tested at different concentrations using an MTT assay (IC50 of 65µg/ml). The E2 KG extract exerts a cytotoxic effect on the U87 glioblastoma cell line, through the negative regulation of cell proliferation pathways: P-PDK1 and P-Akt are both down regulated upon treatment by E2. KG could also play a role in autophagic processes by converting LC3-I in LC3-II. Both electron and confocal microscopy support the implication of E2 in autophagy. The effect on E2 on other cell lines will be studied in the future. This study brings results supporting the effects of Klainedoxa gabonensis Pierre ex Engl., in the treatment of cancers in traditional Congolese medicine.