Concentrated growth factor (CGF) is an autologous leukocyte-rich and platelet-rich fibrin (L-PRF) biomaterial termed "second-generation platelet concentrate". CGF contains autologous osteoinductive platelet growth factors and an osteoconductive fibrin matrix. The purpose of this study was to assess the ability of CGF combined with bone marrow stromal cells (BMSCs) to heal critical-size rat calvaria defects in vivo and to modulate the proliferation and osteogenic differentiation of mesenchymal stem cells (MSCs) in vitro. In the in-vivo study, the CGF group regenerated bone better than the control group, and combined therapy with CGF and BMSCs almost completely repaired critical-size bone defects within 12 weeks after surgery. In the in-vitro study, the CGF extract, at concentrations between 1 and 10%, promoted proliferation, osteogenic maturation, and mineralization of hTERT-E6/E7 human MSCs in a dose-dependent manner but had an inhibitory effect at higher concentrations. In conclusion, a CGF extract promoted the proliferation, osteogenic maturation, and mineralization of mesenchymal stem cells in vitro, and combination therapy with CGF and BMSCs resulted in excellent healing of critical-size bone defects in vivo.