In this study, we report the influence of titania nanotubes (TiNTs) dimensions on the adsorption of collagen (COL) and fibronectin (FN), and its subsequent effect on the growth of osteoblasts. TiNTs with different diameters of around 30 and 100 nm were prepared with anodization. The adsorption profiles of proteins and cell behaviors were evaluated using spectrophotometric measurement, immunofluorescence staining, cell viability, and cytoskeleton morphology, respectively. The results showed that although the growth of osteoblasts was highly sensitive to the dimensions TiNTs, the preadsorbed COL and FN could reduce the difference. Molecular dynamics (MD) simulation results confirmed that the main driving force for protein adsorption was the physical adsorption. The TiNTs with bigger dimensions had higher interaction energies, and thus leading to higher proteins (COL and FN) adsorption and obvious influences on cell behaviors. MD simulation revealed that the orientation and conformation of proteins adsorbed onto surfaces of TiNTs was critical for cell integrins to recognize specific sites. When FN molecules adsorbed onto the surfaces of TiNTs, their RGD (Arg-Gly-Asp) sites were easily exposed to outside and more likely to bond with the fibronectin receptors, in turn regulating the cellular behaviors.