Three new diterpenoids with unprecedented carbon skeletons, cinncassiols F (1) and G (2) and 16-O-β-D-glucopyranosyl-19-deoxycinncassiol G (3), a new isoryanodane diterpenoid, 18-hydroxyperseanol (4), six known isoryanodane diterpenoids, 5-10, and a known ryanodane diterpenoid, 11, were isolated from the stem bark of Cinnamomum cassia. Compound 1 possesses an 11,13:12,13-diepoxy-6,11-epoxy:12,13-disecoisoryanodane diterpenoid skeleton bearing ketal and hemiketal functionalities, whereas compounds 2 and 3 feature an 11,12-secoisoryanodane diterpenoid skeleton with an 11,6-lactone moiety. The structures of the four new diterpenoids, 1-4, and their absolute configurations were established using HRESIMS, NMR, ECD, single-crystal X-ray diffraction, and chemical methods. Compounds 2 and 11 significantly inhibited the proliferation of murine T cells induced by ConA.