A new norlupane-triterpene, $3{alpha},11{alpha}$-dihydroxy-20,23-dioxo-30-norlupane-28-oic acid (1), and four known compounds; $3{alpha},11{alpha}$-dihydroxy-23-oxo-lup-20(29)-en-28-oic acid (2), $3{alpha},11{alpha}$-dihydroxylup-20(29)-en-28-oic acid (3), ent-kaur-16-en-19-oic acid (4), and betulabuside B (5) were isolated from the methanolic extract of the leaves of Acanthopanax koreanum. Their chemical structures were elucidated by mass, 1D- and 2D-NMR spectroscopy. To investigate the bioactivities of these compounds, their effects on the differentiation of osteoblastic MC3T3-E1 cells were tested. Compounds 3 and 4 significantly increased osteoblastic cell growth and differentiation as assessed by the MTT assay and collagen content, respectively (p < 0.05). Compounds 1 and 5 significantly increased the growth of MC3T3-E1 cells and caused a significant elevation of osteoblastic cell differentiation as assessed by the alkaline phosphatase activity. These results suggest that the compounds isolated from A.koreanum leaves have a direct stimulatory effect on bone formation in vitro and may contribute to the prevention for osteoporosis.