Steroid and triterpenoid metabolites are a large and structurally diverse group of natural products, and nearly 200 distinct skeletons have been identified.They are synthesized via the 30 carbon intermediate 2,3-oxidosqualene, which is cyclized by members of the oxidosqualene cyclase (OSC) family to different cyclisation products we carry out functional analysis of two oxidosqualene cyclases (OSCs), parkeol and isoarborinol synthases (encoding by Os11g08569 and Os11g35710, respectively) from rice, and show that they produce the triterpenes parkeol and isoarborinol in plants, respectively.Our systematic phylogenetic analysis of plant OSCs, including OSCs from rice and a range of higher and lower plants, suggests that distinct plant lineages are likely to produce different triterpene skeletons and redundant OSC genes were rarely found in the same genome.lnterestingly, dicot triterpene synthases were derived from an ancestral lanosterol synthase instead of from the ancestral cycloartenol synthases.We also found out that expansion of OSC gene family in higher plants was mainly through tandem duplications following with subsequent fast sequence divergence.The easiness in generation and flexibility in adaptation of tandem duplicates may provide great evolutionary advantages in generation of diversified metabolites in responding to environmental stresses.