Siegesbeckia pubescens (S. pubescens) was widely used to alleviate symptoms of osteoarthritis (OA) in traditional medicine. Three new ent-kaurane-type diterpenoids, siegesbeckioside, siegesbeckiol and siegesbeckic acid, were isolated together with five known diterpenes from Siegesbeckia pubescens (Xi-Xian). On the basis of chemical and spectral evidences, their structures were elucidated as ent-16β,17,18-trihydroxy-kauran 18-O-β-d-glucopyranoside, ent-kauran-2α,16β,17-triol, ent-16α,17,18-dihydroxy-kauran-19-oic acid, ent-kauran-16β,17,18-triol, ent-16β,17-dihydroxy-kauran-19-oic acid, ent-16α,17-hydroxy-kauran-19-oic acid, grandifloric acid and kirenol, respectively. A novel HPLC method was developed for the simultaneous quantification of seven major diterpenoids of two types, including ent-pimarane type: Kirenol, Hythiemoside B, Darutigenol, and ent-kaurane type: ent-16β,17,18-trihydroxy-kauran-19-oic acid, ent-17,18-dihydroxy-kauran-19-oic acid, ent-16β,17-dihydroxy-kauran-19-oic acid, 16α-hydro-ent-kauran-17,19-dioic acid in the aerial parts of Siegesbeckia pubescens.