Eight new ent-pimarane diterpenoids and diterpenoid glycosides, namely, ent-12alpha,16-epoxy-2beta,15alpha,19-trihydroxypimar-8-ene (1), ent-12alpha,16-epoxy-2beta,15alpha,19-trihydroxypimar-8(14)-ene (2), ent-2alpha,15,16,19-tetrahydroxypimar-8(14)-ene, ent-15-oxo-2beta,16,19-trihydroxypimar-8(14)-ene, ent-2-oxo-15,16-dihydroxypimar-8(14)-en-16-O-beta-glucopyranoside, ent-2-oxo-15,16,19-trihydroxypimar-8(14)-ene, ent-2-oxo-3beta,15,16-trihydroxypimar-8(14)-en-3-O-beta-glucopyranoside, and ent-2beta,15,16,19-tetrahydroxypimar-8(14)-en-19-O-beta-glucopyranoside, together with seven known diterpenes were isolated from Siegesbeckia orientalis. Compounds 1 and 2 are novel ent-pimarane diterpenoids with an unprecedented 12alpha,16-epoxy group. Their structures were established by spectral methods, especially 1D and 2D NMR spectral methods.