Rhododendron molle G.Don is first recorded in Shengnong's Herbal Classic,and its fruits,which are termed as Liuzhouzi,are often used to treat rheumatoid arthritis in Chinese folk.During our ongoing investigation to develop a safer and potential new arthritis therapy,a process for the preparation of diterpenoid fraction from Rhododendron molle fruits was established.In order to evaluate the main components and the anti-rheumatoid arthritis effect of the diterpenoid fraction,phytochemical and pharmacological experiments were used.As the result,the main components of diterpenoid fraction were identified as rhodojaponin Ⅲ (1),rhodojaponin Ⅵ (2),2-O-methylrhodojaponin (3),and 5'-β-D-glucopyranosy-loxyjasmonic acid (4).These four components constitute greater than 95％ of diterpenoid fraction using area normalization method of HPLC-ELSD.The results of CIA rat experiment showed that high dose of diterpenoid fraction (0.6 mg.kg-1.d-1) significantly alleviated the symptoms of rheumatoid arthritis,similar to tripterygium polyglycosides,an effective RA therapy.Preliminary mechanism studies indicated that diterpenoid fraction significantly inhibited the abnormal proliferation of T and B lymphocytes,and remarkably reduced the levels of pro-inflammatory cytokines IL-6,IL-1β and TNF-a.Overall,our findings may provide a more effective and safe alternative treatment for RA using common clinical Chinese medicines like tripterygium polyglycosides.