Poor interaction between cells and surfaces, especially hydrophobic surfaces, results in delayed proliferation and increased apoptosis due to low cell adhesion signaling. To improve cell adhesion, hydrophilic array of amorphous calcium phosphate (ACP) was fabricated on a surface. A phosphate-buffered solution containing calcium ions was prepared at low temperature to prevent spontaneous precipitation. Then, the ion solution was heated to generate nuclei of ACP nanoparticles. The ACP nanoparticles adhered to the hydrophobic polystyrene surface forming an array composed of ACP particles. Multiple treatments of these nuclei with fresh CaP ion solutions increased the diameter and decreased the solubility of ACP particles enough to mediate cellular adhesion. The particle density in the array was dependent on the ion concentration of the CaP ion solutions. The ACP array improved a wide variety of activities when osteoblastic MC3T3-E1 cells were cultured on the ACP array fabricated on a hydrophobic bacteriological dish surface, compared to those cultured without the ACP array in vitro. The use of ACP array resulted in a lower apoptosis and also increased the spreading of cells to form stress fibers and focal contacts. Cells cultured on the ACP array proliferated more than cells cultured on a hydrophobic surface without the ACP array. The ACP array increased the expression of markers of differentiation in osteoblast. These results indicate that an array of ACP can be used as a coating material for enhancing biocompatibility in tissue engineering or biomaterials rather than modifying the surface with organic molecules.