RF sputtering process was applied to produce thin hydroxyapatite[HA, Ca10(<TEX>$PO_4$</TEX>)<TEX>$_{6}$</TEX> <TEX>$ (OH)_2$</TEX>films on Ti-6Al-4V alloy substrates. To make a 101.6 mm dia.<TEX>${\times}$</TEX>5 mm HA target, the commercial HA powder was first calcinated for 3h at <TEX>$200^{\circ}C$</TEX>. A certain amount of the calcinated HA powder was pressed under a pressure of 20,000 psi by the cold isostatic press(CIP) and the pressed HA target was sintered for 6 h at <TEX>$1,200^{\circ}C$</TEX>. The effects of different heat treating conditions on the bonding strength between HA thin films and Ti-6Al-4V alloy substrates were studied. Before deposition, the alloy substrates were annealed for 1 h at <TEX>$850^{\circ}C$</TEX> under <TEX>$3.0<TEX>${\times}$</TEX>10^{-3}$</TEX> Xtorr, and after deposition, the hydroxyapatite/Ti-6Al-4V alloy thin films were annealed for 1 h at 400, 600 and <TEX>$800^{\circ}C$</TEX> under the atmosphere, respectively. Experimental results represented that the HA thin films on the annealed substrates had higher hardness than non-heat treated substrates before the deposition.