The title xanthone known as Formoxanthone C (1) was isolated from the green fruits of Cratoxylum formosum ssp. pruniflorum. Compound 1 crystallized out in P2(1)/c space group, indicating that 1 exists both enantiomers. The unit cell parameters are a = 13.8038(2), b = 14.6712(2), c = 9.4686(1) angstrom, beta = 95.358(1)degrees, V = 1909.19(4) angstrom(3), and Z = 4. The alpha,alpha,beta-trimethyldihydrofuran ring was in an envelope conformation and disordered over two sites with 0.628(1) and 0.372(1) occupancies for the major and minor components, respectively, which corresponded to the less furan ring strain and the steric hindrance between the two methyl substituents (at positions 1 ' and 2 ') on the furan ring of the major component than that of the minor component. The crystal structure is stabilized by intermolecular O-H center dot center dot center dot O hydrogen bonds and weak C-H center dot center dot center dot O interactions.