Four sesquiterpenoid derivatives, i.e., 4β,10α-dihydroxyaromadendrane (1), caryophyllenol-II (2), senecrassidiol (3), and clovane-2β,9α-diol (4) have been isolated from the stem bark of Aglaia simplicifolia. The chemical structures of compounds 1-4 were determined based on spectroscopic data, including one and two-dimensional NMR and mass spectroscopy. In addition, these sesquiterpenoids 1-4, were also tested for their cytotoxic activity against B16-F10 melanoma skin cancer cell lines through in vitro assay. Among the isolated compounds 1-4, compound 1 showed the highest activity with an IC50 value of 44.8 μg/mL, suggesting the presence of a cyclopropane ring that plays an essential role in cytotoxic activity against B16-F10 melanoma skin cancer cell lines.