This study investigated the chemical composition (by using gas chromatography/flame ionization detection and gas chromatography/mass spectrometry, an antioxidant [1,1-diphenyl-2-picrylhydrazyl] [DPPH] radical-scavenging assay, and a 2,2'-azinobis-3-ethylbenzothiazoline-6-sulfonate [ABTS] radical cation-scavenging assay) and the antimalarial and cytotoxic activities of essential oil extracted from leaves of Melaleuca armillaris. Thirty-two components representing more than 98% of the total composition of the essential oil were identified. The main components were 1,8-cineole (85.8%), camphene (5.05%), and α-pinene (1.95%). The antioxidant activity by ABTS assay showed a mean (± standard deviation) 50% inhibitory concentration (IC(50)) value of 247.3 ± 3.9 mg/L, and the DPPH assay yielded an IC(50) value of 2183.6 ± 44.3 mg/L. The antimalarial study indicated that the essential oil had mild activity against the chloroquine-resistant Plasmodium falciparum FcB1 strain (IC(50), 27 ± 2 mg/L). The cytotoxic activity of this essential oil was tested against MCF7 human breast cancer cells and was found to be high (IC(50), 12 ± 1 mg/L).