The analysis of co-culture system using osteoblast and bone marrow indicated that the mineralized osteoblast decreased osteoclast formation. This finding was an incentive to better investigate the relation of osteoblast development and osteoclastogenesis. The expression of osteoclast differentiation factor (ODF/ RANKL) mRNA and protein dramatically decreased. Alternatively, macropharge colony stimulation factor (M-CSF/CSF-1) transcription and protein secreted in media slightly decreased as the development of osteoblast. On the other hands, mRNA expression and the secretion to the culture medium of osteoclastogenesis inhibitory factor (OPG/OCIF) didn't significantly change depending on osteoblast differentiation. We conclude that osteoblast development might suppress osteoclastogenesis especially with the decrease of ODF/RANKL.