To compare the effects of Tannin-fluoride preparation (HY agent) on the inorganic phases of the teeth, with those of ZnF2 and SrF2 which are components of the HY agent, we immersed synthetic hydroxyapatite (HAp) to the saturated solution of each fluoride and the HY agent for 1 day and 10 days, and studied the fluoride uptake of the HAp, changes in the acid resistance of the HAp and X-ray diffraction patterns of the HAp. In addition, we also compared them with the NaF solution which has the same fluorine concentration as the HY agent solution. The HAp immersed in the ZnF2 solution had the largest fluoride uptake, and superior to the others with respect to the acid resistance. It was thought that zinc phosphate and calcium fluoride which were recognized by X-ray diffraction analysis were effective on the acid resistance. Fluorine concentration of the SrF2saturated solution was about 1/100 of the other solutions, though the fluoride uptake of the HAp immersed in the SrF2 solution was 1/3 to 1/6 of the other samples. The acid resistance of the SrF2 sample was about 1/2 of the ZnF2 sample, but it was not different significantly from the HAp immersed in the NaF solution. The fluoride uptake of the HAp immersed in the HY agent solution was a little less than that of the ZnF2 sample. The acid resistance of the HAp immersed in the HY agent solution for 10 days was not different significantly from both the HAp immersed in the SrF2 solution and the NaF solution. Consequently, though the HY agent contains the tannic acid which converts collagen to collagenase-resistance forms, it was clear that the ZnF2 contained in the HY agent had great importance with respect to the increase of acid resistance.