The water behavior on (001) and (100) crystal faces of hydroxyapatite (HAP) were studied using molecular dynamics (MD) simulations.The study showed that the water molecules between the HAP faces were under conditions of strong electrical field and high pressure,and hence formed 2-3 well-organized water layers on the crystal surfaces.These structured water layers had ice-like features.Compared with the crystallographic [100] direction of HAP,the polarity along the [001] direction was stronger,which resulted in more structured water layers on the surface.The interaction of water molecules with the calcium and phosphate sites at the HAP-water interface was also studied.The results indicated the multiple pathways of water adsorption onto the HAP surfaces.This study revealed the formation and the detailed structure of water layers on HAP surfaces and suggested that the interracial water played an important role in stabilizing the HAP particles in aqueous solutions.