Marcanine A was isolated from the stems of Polyalthia plagioneura as light yellow crystals. The molecular and crystal structures have been determined by 1D,2D-NMR and X-ray diffraction analysis. It crystallizes in the triclinic system, space group P-1 with a = 5.2140(5)Å, b = 10.1871(11)Å, c = 11.0709(13)Å, α = 110.452(2)º, β = 103.376(2)°, γ = 90.1870(10)°, V = 533.74(10)A3, Z = 2. There are three intermolecular hydrogen bonds in a unit cell. It displays some inhibitory activities towards four kinds of human tumor cells, including BEL-7402, K562, SPCA-1 and SGC-7409.