The use of carbon nanotubes for tissue engineering has become one of the most fascinating applications. The exquisite electronic and mechanical properties of carbon nanotubes may provide a three-dimensional (3D) microenvironment that closely mimics in vivo situation for facilitating the use of stem cells in the tissue regeneration. Therefore, it is important to know whether carbon nanotubes enhance the adhesion, proliferation, and differentiation of stem cells. Here, we hypothesized that the carbon nanotubes promote the differentiation of osteoblast progenitors into mature osteoblasts. To test this hypothesis, we quantified the differentiation of murine osteoblast progenitors, with and without pro-differentiating growth factor Bone Morphogenetic Protein-2 (BMP-2), cultured on the 3D scaffolds made by single-walled carbon nanotubes (SWNTs). Three types of SWNT samples, chitosan functionalized SWNTs, acid-oxidized SWNTs, and surfactant-free pristine SWNTs were used for the construction of these 3D microarchitectures. Osteoblast progenitors were harvested from calvariae from 3~5-day-old mice, and cultured on the 3D scaffolds made by carbon nanotubes until ~ 80% confluent. Then the cells were treated with BMP-2 (100 ng/ml) for 5 days. It was found that osteoblast progenitors cultured on the SWNTs dramatically increased the level of mature osteoblastic marker osteocalcin in either the absence or the presence of BMP-2, as compared to the cells cultured on the regulate tissue culture plastic plates. The results suggested that SWNTs highly promote osteoblast progenitor differentiation into mature osteoblasts. The finding indicates that SWNTs may provide an ideal scaffold for facilitating the differentiation of osteoblast progenitors in the repair of bone defects.