Objective: To investigate the effects of superhydrophobic modification of hydroxyapatite on the adhesion of Streptococcus mutans and calcium dissolution. Methods:The surface of hydroxyapatite (HAP) discs were modified with a self-assembling film of fatty acids (FAs), CH3(CH2)n-1COOH in ethanol, with different carbon chain lengths (n = 1, 2, ... , 16) for 12 hrs to obtain superhydrophobicity. The water contact angle on the surface of the modified HAP discs was measured to determine the superhydrophobicity. The superhydrophobically modified HAP discs were coated with human saliva, incubated in a S. mutans suspension for 24 hrs and the amount of the bacterium bound to the HAP disc surface was measured as colony-forming units. The HAP discs modified with FAs with different carbon chain length were also examined for their resistance to acid by immersing in an artificial caries demineralisation solution for 12 hrs. The calcium dissolved in the demineralisation solution was measured by atomic absorption spectroscopy. Further, the resistance of superhydrophobically modified HAP discs to acid was examined by the same method at 1, 8, 24 and 48 hrs. Results: The contact angle increased significantly with the increase of the FA carbon chain length, and the surfaces of the modified HAP discs became superhydrophobic (contact angle larger than 150 degrees) when the FA carbon chain length was 12 or more. The amount of bacteria that adhered to the superhydrophobically modified HAP discs was significantly less than the control group (P 0.05). The calcium dissolved from the FA-modified HAP discs into the demineralisation solution was dramatically decreased when the FA carbon chain length was 12 or more. The superhydrophobically modified HAP discs had less calcium dissolved into the demineralisation solution compared with the control (non-modified) for up to 48 hrs (P 0.01). Conclusion: HAP discs were easily superhydrophobically modified by FAs. The superhydrophobic modification efficiently reduced both the oral bacterial adhesion to the surface of the modified HAP discs and calcium dissolving from the modified HAP discs. This is likely to be of significant interest in caries prevention. Keywords: acid erosion, bacterial adhesion, hydroxyapatite, superhydrophobicity