Recently, nanomaterials have received considerable attention because of their potential applications in the biomedical field. In the present study, we investigated the effects of nano-sized calcium metaphosphate (CMP) particles (50 nm) compared with micro-sized CMP particles (200-500 nm and 10 μm) on the proliferation and osteoblastic differentiation of human bone marrow stem cells (BMSCs). BMSCs were challenged with CMP particles with different sizes for 3, 5, and 7 days. An analysis of the proliferation revealed that the nano-sized CMP particles (50 nm) stimulated the proliferation of BMSCs up to 27.79% compared to the untreated control. This stimulatory effect of the nano-sized CMP particle was dose-dependent. CMP particles appeared to adhere on the surface of BMSCs but this did not cause distinguishable morphological changes. Moreover, all CMP particles (50 nm to 10 μm) were capable of stimulating an osteoblastic differentiation of BMSCs as accessed by alkaline phosphatase (ALP) and von Kossa stainings. Further molecular analysis revealed that all the CMP particles induced an expression of osteoblast-related genes such as osteocalcin (OC) and collagen I (Col I). Taken together, our data demonstrate that nano-sized CMP particles have the potential to stimulate the proliferation and osteoblastic differentiation of BMSCs.