Hydroxyapatite (HA) coated total hip joint device has caused concerns of generating wear particles after long-term implantation. We designed a pin-on-disk (POD) test to examine the morphological changes and wear particle generation of plasma sprayed HA coating in vitro. HA coatings were immersed in supplemented α-calf bovine serum for 48 hours. Serum soaked HA coating exhibited significant amount of weight loss due to dissolution of amorphous calcium phosphate (ACP). POD test demonstrated the serum soaked HA coatings presented many micronsized particles on the surface while the as-received HA coatings maintained good integrity. The generation of wear particles of the serum soaked HA coatings is related to the reduction of the cohesion/adhesion of HA coatings due to the preferential dissolution of ACP.