To increase the mechanical property of zirconia, we have investigated the phase change and the resulting hardness of zirconia ceramics by hydroxyapatite (HA) powder bed sintering. It was observed using X-ray diffraction that the cubic zirconia phase, which has a higher hardness value than that of the tetragonal phase, was obtained at the surface of 3 mol% $Y_2O_3$ doped tetragonal zirconia polycrystal (3Y-TZP) ceramics during the sintering process; in our experimental conditions, the phase change at the surface increased as the sintering time increased. We believe that the observed crystalline phase change originated from the decomposition of HA and the diffusion of CaO, as follows. CaO, which was derived from the decomposition of HA at high temperature ($1400^{circ}C$), diffused into the surface of 3Y-TZP and acted as a stabilizer. As a result, the Vickers hardness value of the treated specimens was higher than that of the non-treated specimen due to the formation of the cubic phase on the surface of 3Y-TZP.