The ethanolic extract of the leaves of Alstonia boonei yielded yellow oil (2.38g). The oil was subjected to GC-MS studies. Fifteen phyto-constituents were identified with Ascorbic acid 2,6-hexadecanoate (17.27%) constituting the bulk of the oil, followed by 9-Octadecenoic acid, 1,2,3-propanetriyl ester (12.89%). Other monoterpenoids, oxygenated compounds and fatty acids identified include Octadecanoic acid (10.31%), Octadecanoic acid methyl ester (9.02%), Benzoic acid thio (7.73%), 9-Octadecene (E) (6.44%), 2,3-Diazabicyclo [2.2.1] hept-2-ene, 4-methyl-1-(Pent-4-en-1-yl) (5.15%), 1,2-Oxathiane, 6-Dodecyl-2,2-dioxide (3.87%) while 1-Undecene-9-methyl and 1-Heptanol-6-methyl respectively contained 2.58%. n-Decanoic acid contained 1.55% while 1-Hexanol, 5-methyl and Bicyclo [3.1.1] heptane 2,6,6-trimethyl constitutes 1.29% of the oil respectively. The volatile oil showed antibacterial activity against Escherichia coli, Streptococcus pneumoniae, Staphylococcus aureus and Proteus mirabilis. These results lend credence for the use of oil extract from the leaves of Alstonia boonei in the treatment of asthma, coughs and wounds in herbal medicine in Nigeria.