Hydroxyapatite (HAP) thin films (0.2 μm, 0.3 μm and 1.2 μm thickness) were grown by RF magnetron sputtering in argon atmosphere onto silicon substrates at high deposition rates (0.6 μm/h). Crystalline HAP films were obtained using a low temperature (100 °C) followed by post-deposition annealing at 300 °C, 450 °C, 500 °C and 550 °C in environmental air for 1 hour. An important influence of the films thickness upon the crystallization degree was noticed at intermediate annealing temperatures, as obtained from XRD measurements. For low and high temperatures similar values were obtained with a better crystallization degree for the thinner films. FTIR absorption led to the same conclusion considering the shape of stretching and bending PO 4 lines. This suggests that the crystallization process has a diffusion component besides usual thermal activation process.