A new controlled release system is proposed for bone morphogenetic proteins (BMP)-induced osteoblastic growth and differentiation of the human bone marrow stromal cells (hBMSCs) using injectable nanohydrogels. Alginate nanohydrogels (ANH) containing BMP-2 (BMP-2@ANH) were prepared using a reverse emulsificationdiffusion technique. The mean particle diameter of the BMP-2@ANH measured by dynamic light scattering was 520 nm. The proliferation of hBMSCs was promoted with the BMP-2@ANH. The controlled release of BMP-2 from the BMP-2@ANH enhanced the differentiation of hBMSCs into osteoblasts, which was confirmed by RT-PCR and alkaline phosphate activity. With the BMP-2@ANH, calcification and phosphate deposits on the cells were confirmed by alizarin red-S staining and von-kossa staining, respectively, showing that the differentiation of hBMSCs into osteoblasts was activated by the BMP-2 released from the BMP-2@ANH. Growth factor delivery using a nanohydrogel is a potential method for facilitating the differentiation of stem cells in vivo.