Seven new diterpenes, named ovoideal A (1), B (2), C (3), D (4), E (5), F (6) and G (7), have been isolated along with eleven known diterpenes 8-18 from the petroleum ether soluble fraction of an ethanol extract of the aerial parts of Tirpitzia ovoidea. The structures of the new compounds were elucidated primarily by 1D and 2D NMR spectroscopy, as well as by the HR-ESI-MS spectrometry. All compounds were isolated from the Linaceae family for the first time. The in vitro cytotoxic activity of compounds 1, 3-5, 8-18 was evaluated against the Hela, HepG2 and K562 cell lines. Among them, compounds 3, 9, 11, 12, 13, 14, 15, 17, 18 showed moderate inhibitory activities.