Five new cembrane-type diterpenes (1–5), one new casbane-type diterpene (7), along with ten known related ones (6, 8–16), were isolated from the South China Sea soft coral Sinularia nanolobata. The structures of new compounds were elucidated through comprehensive spectroscopic analysis, NMR calculation with DP4+ probability analysis, time dependent density functional theory-electronic circular dichroism (TDDFT-ECD) calculations, and comparison with the reported spectroscopic data of known analogues. Structurally, compounds 2–5 featured rare five- to seven-membered oxygen-containing heterocyclic rings with diverse joints, reflecting remarkable chemical diversity of secondary metabolites. In bioassays, all isolates have been evaluated for their cytotoxic and anti-inflammatory effects.