The present study examines the effect of residual stresses in different coating thickness on the shear strength of hydroxyapatite coating (HAC) to bone. Plasma-sprayed HAC 50 and 200μm thick on Ti6Al4V cylinders was transcortically implanted in the femora of canines to evaluate the push-out shear strength after the periods of 4, 6, 8 and 12 weeks. Push-out testing of implant-bone interfaces showed that the HAC coating exhibited higher shear strength at 50μm than 200μm HAC (13.97 vs. 9.24 MPa after 12 weeks implantation). From the stress measurement, the plasma-sprayed HACs were exhibited compressive residual stresses and the major principal stresses for 200μm and 50μm HACs were in compression with 17.1 and 14.9 MPa. Significantly, the thicker HAC exhibited higher residual stress than that of the thinner HAC. According to the failure mode of 50μm HAC, the slight attack of body fluid might not degrade the implant to the extent that failure occurred at the HAC-Ti alloy interface after 12 weeks. Therefore, the variation of failure mode of 200μm HAC with time could not be accounted for by the attack of body fluid. From the literature, in-plane compressive residual stress would induce the through-thickness tensile stress acting in the direction normal to the HAC-Ti interface and that is likely to weaken the adhesion strength between the HAC and Ti-substrate. Moreover, the structure for 50 and 200μm implants were the same, meaning similar cohesive strength of the lamellar splats. And, there was no difference in the physiological environment; hence the difference of the shear strength for the 50 and 200μm implants could best be attributed to the compressive residual stress. In conclusion, the thicker 200μm HAC exhibited higher residual stress and poor mechanical stability in vivo, as compared with the thinner 50μm HAC having lower residual stress and superior performance.