Biogenesis-inspired chemical research of the leaves of Taiwania cryptomerioides afforded four unprecedented dimeric diterpenes, featuring a tetracyclic [7. 7^{5, 9}. 4. 0^{5, 10}. 0^{8, 9}] octodecane core: taiwanoids A–D (1–4). The structures of these compounds were determined on the basis of comprehensive spectral analysis, chemical conversions and X-ray crystallography. A possible biosynthetic pathway for compounds 1–4 was proposed. Compounds 2 and 3 exerted a 5.37 and 6.26-fold potentiation effect on bortezmib (BTZ) susceptibility at a tested concentration of 20 μM, respectively.