Dentin is a load bearing multiphase composite composed of a ceramic phase, hydroxyapatite (HAP), a polymeric phase, collagen, and fluid filled porosity. In order to create better dentin replacements it is important to understand how applied load is naturally transferred between the phases during chewing and other stresses. To determine the apparent elastic modulus of HAP in dentin, applied stress over lattice strain in HAP, high energy wide angle x-ray diffraction measurements were performed on in situ loaded bovine dentin samples. It was determined that the average longitudinal apparent elastic modulus of HAP in dentin was 18.3±2.19GPa. This value is much lower than values predicted by the Voigt model when combined with volume fractions determined for the sample by thermo-gravimetric and chemical analysis. It has been determined that the decrease in apparent elastic modulus is most likely due to a decrease in the "bulk" elastic modulus of HAP due to nanometric effects.