One of the major features of dentin structure is microtubules which pass through the matrix formed by collagen fibrils and hydroxyapatite nanocrystals. The effect of the microtubules on the fracture toughness of dentin has been studied. The samples, prepared from human molars, were subjected to uniaxial compression until cracking. Then these samples were investigated by synchrotron radiation (SR) microtomography. The results obtained allowed us to work out a model of the mode II crack development in human dentin.