The precipitated amorphous calcium phosphates (ACP) were modified by stearic acid (SA) at about 78℃ for different times, using ethanol as reaction medium in a reflux setup. Modification effects of SA were investigated by activation ratio, contact angle, FTIR and TGA/DTA. The stability of the ACP powders before and after modification was examined by FTIR, XRD and TGA/DTA. The results show that the optimal adding amount of SA is 5 wt%, there are about 4 wt% SA on the surface of the ACP powders, and SA reacts with ACP to form -COOCa and HPO₄^2- groups which leads to a hydrophobic coating form on the ACP surface containing the long alkyl chain of SA. The ethanol mediated reflux method does not induce phase conversion of ACP at about 78℃ for 24h, and the modified ACP powders are still amorphous even after stored in air at room temperature for more than one year. It is proposed that SA existing on the surface of the ACP powders has higher ability to retard conversion of ACP into its crystalline polymorphous than β-CD due to the strong steric hindrance of the long alkyl chain of SA.