We observed a critical phenomenon of ferroelectric transition in hydroxyapatite (HAp) through measuring the thermally stimulated depolarization current of polycrystalline monoclinic HAp. Two peaks attributable to hydroxide (OH-) ion dipole reorientations were observed. The one was very sharp, the other displayed a broad peak ranging from 350 to 550K. While the broad peak can be attributed to the diffusive reorientation motions of OH- ion dipoles, the sharp peak, related to the ferroelectric phase transition, implies the presence of ferroelectricity in HAp. The sharp peak was found near 380K, the phase-transition temperature from the monoclinic to hexagonal is ascribed to a critical phenomenon caused by the ferroelectric behavior of OH- ion dipole reorientations.