The effect of triptolide (TP) on the expression of receptor activator of nuclear factor-κB ligand (RANKL) and osteoprotegerin (OPG) was explored in rat adjuvant induced arthritis (AA).AA was induced in Wistar rats. Arthritis rats were treated with TP and methotrexate (MTX) at the onset (day 9) of arthritis. On the peak of arthritis (day 24), the expression of RANKL and OPG protein in the joints and RANKL mRNA in peripheral blood mononuclear cells (PBMC) was detected. TNF-α and IL-1β levels in peripheral blood were determined, Bone erosion scores were also evaluated. The results showed that bone erosion scores in TP and MTX groups were lower than in AA group (P＜0.01); The expression levels of RANKL in the synovium (P＜0.01) and bone (P＜0.05), and OPG level in synovium (P＜0. 05) were lower in TP group than in AA group (P ＜0.05). In TP group, the expression levels of RANKL mRNA and TNF-α, IL-1β in PBMC were lower than in AA group (all P＜0.01). It was concluded that TP could inhibit rat adjuvant arthritis bone erosion by suppressing the expression of RANKL.