Chemical composition and biological activities of the essential oil of Detarium microcarpum leaves were assessed in vitro. The essential oil was obtained using a Clevenger type apparatus and analyzed by GC and GC/MS. The essential oil was investigated for antioxidant power using the DPPH radical scavenging method and the FRAP test. Antimicrobial activity of the essential oil was also evaluated by agar disk diffusion and broth micro dilution methods. Chemical analysis showed that the major components of the essential oil of D. microcarpum were caryophyllene oxide (28.186%), β-caryophyllene (11.894%), humulene-1, 2-epoxide (4.05%), neryl acetone (2.452%), salvia-4(14)-ene-1-one (2.423%) and linalol (2.098%). The essential oil showed a low radical scavenging power with an IC50 of 21.99 ± 0.17 µl and a weak reducing power. The essential oil exhibited strong to moderate antimicrobial activity against Bacillus subtilis, Escherichia coli, Bacillus cereus, Clostridium perfringens, Enterococcus faecalis, Listeria monocytogenes, Micrococcus luteus, Staphylococcus, Candida kefir and Saccharomyces cerevisiae with inhibition zone diameters ranging from 12 ± 0.00 mm to 22 ± 1.41 mm. Minimum inhibitory concentrations of the essential oil were between 0.5% and 8% (v/v) with minimum bactericidal concentrations of 1% to 8% and minimum fungicidal concentrations of 1% to 8%. The leaves essential oil of D. microcarpum had an absolute bactericidal action against Staphylococcus aureus ATCC2523. This study provided data on the proximate composition and biological activities of the essential oil of D. microcarpum leaves and these results confirmed some traditional usage of this plant. The essential oil of D. microcarpum could be a good candidate in the research of novel antimicrobial compound.