The idea of using nanocrystalline diamond (NCD) as a coating on orthopedic implants originates back to the last century since NCD possesses superior mechanical and tribological properties as well as chemical stability. However, it has only been within recent years that the interactions between NCD and osteoblasts (OB, bone forming cells) have been investigated. In this study, the impact of NCD surface topography on OB functions including proliferation (24 hrs to 48 hrs) and differentiation (7 to 21 days) was studied. NCD of varied grain sizes (from less than 100 nm to approximately 600 nm) were fabricated by microwave plasma enhanced chemical-vapor-deposition and characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Surface topography of the NCD changed dramatically as grain size grew. OB proliferation on these coatings was studied by SEM after incubation for 24 hrs and 48hrs, respectively. OB differentiation on diamond coatings after incubation from 1 to 3 weeks was investigated by alkaline phosphatase activity and calcium deposition. Results demonstrated that OB proliferation and differentiation were dependent on topography with NCD grain sizes less than 100 nm exhibiting the best OB responses. To explain this enhancement, OB filopodia protrusions on different NCD were observed by SEM and the results revealed that surface topography of NCD played a crucial role in OB filopodia extensions. In summary, these findings provided important design criteria for creating proper NCD surfaces for orthopedic coatings and also provided cues on promoting interactions between nanostructured surfaces and cell responses.