OCTACALCIUM PHOSPHATE (Ca8H2(PO4) 6·5H2O; OCP) is considered as a precursor of biological apatite crystals in bone and tooth in vertebrates. Recent studies have suggested that osteoblastic cell differentiation is enhanced in contact with OCP which is converted into HA and is significantly facilitated in an OCP dose-dependent manner. MAPK signaling is known to be activated in the early stage of the differentiation from bone marrow-derived mesenchymal stem cells into osteoblasts. However, the MAPK signaling is unclear in relation to the stimulatory capacity of OCP in the osteoblastic differentiation. In this study, we investigated the MAPK signaling during the differentiation of bone marrow-derived stromal cell line leading to the late differentiation stage using gene expression analysis.