Synthetic polymer (RADA16:PuraMatrix®) is an artificial peptide and self-assembling. RADA16 is able to provide three dimensional constructs which hold mesenchymal stem cell (MSCs). We combined the MSCs into the polymer and fabricated MSC/RADA composites. The composites showed osteogenic differentiation in the culture condition. Supplementation of hydroxyapatite ceramics (HA) in the composites (MSC/RADA/HA composites) improved the mechanical property and demonstrated the extracellular bone like matrix formation in the culture condition. These results indicate the important property of HA ceramics for the proliferation and differentiation of MSCs in the polymer. The MSC/RADA/HA composites could be available in the field of bone tissue engineering.