The response of osteoblast-like cells seeded on hydroxyapatite (HAp) substrates consisting of nanosized crystals was investigated. Various types of HAp nanocrystals, such as nanofibers, nanoneedles and nanosheets, were selectively prepared as substrate through the hydrolysis of a solid precursor crystal of CaHPO(4) in alkaline solutions by varying the pH and ion concentrations. Although all the substrates were macroscopically flat and smooth, the nanoscale topography influenced cell activity, including the adhesion, proliferation, elongation and formation of actin stress fibers. The presence of fine nanoneedles and nanofibers on the surface restricted the cellular activities, while the cells steadily proliferated on a nanoscopically smooth surface of large grains and on a substrate consisting of wide nanosheets. These results suggest that the adhesion and subsequent responses of osteoblast-like cells were affected by the contact domain size between the cell and the substrate. Isolated small domains of the nanostructured HAp limited focal adhesion formation in the cells associated with the formation of stress fibers. Stable adhesion with contact domains larger than 100nm in width was suggested to be required for cell survival. On the other hand, insufficient adhesion on the fine nanoneedles was found to lead to apoptosis.