Heartwood of Caesalpinia sappan L. has been traditionally used to many diseases such as homoptysis, syphilis, eye disease, dysentery, depurative and prevention of osteoporosis. Our previous in vitro screening of Indonesian plants revealed that an ethanolic extract of the heartwood of C. sappan exhibits a proliferation stimulating activity against primary osteoblastic cells. In our continued interest to this plant, we further fractionated the extract and isolated active constituents on the basis of the stimulating activity in the osteoblastic cells. The fractionation and isolation were carried out with various chromatography methods and the structure of isolated compounds was elucidated based on NMR, IR, UV and MS spectroscopic data. From an active fraction, a new biphenyl dimer, namely caesappanin C (1), along with two known compounds, protosappanin A (2) and sappanchalcone (3), were isolated. Among them, the new compound 1 exhibited the strongest activity and significantly increased the cell viability up to 276±5%. The other two compounds 2 and 3 also stimulated the cell proliferation and increased the cell viability up to 233±8% and 187±4%, respectively.