The chemical compositions of essential oils obtained by hydrodistillation from leaves and stems of Xyloselinum vietnamense and X. leonidii, two new species belonging to the family Apiaceae, were analyzed by GC-MS. The major components in both species were sabinene, alpha- and beta-pinene, myrcene, beta-phellandrene, (Z)-beta-ocimene, and terpinen-4-ol. The monoterpene sabinene was most abundant in the leaves of X. vietnamense (75.0%). These compounds might be considered as chemotaxonomic markers of Xyloselinum species. In the DPPH radical scavenging assay, all four essential oils showed moderate activity, while the water extracts exhibited stronger effects. The strong DPPH scavenging activity of the water residues of X. vietnamens and X. leonidii might be due to their phenolic components. This paper is the first report on the chemical compositions and antioxidant activity of X. vietnamens and X. leonidii.