A structure and a crack growth mechanism of thin samples of teenage crown dentin on micro- and nanoscales have been studied. It was found that on the microscopic level the thin samples of crown teenage dentin failed as provided by the viscoelastic manner like thin samples of mineralized mature dentin. On the nanoscale level the crown teenage dentin has a layered structure (a single layer has 50—100 nm in thick and lays normally to the main axis of a tooth) and behaves like a viscoelastic solid, such as some polyethylene films and rubbers when a crack starts to move after considerable elastic and irreversible deformation. Hence, the viscoelastic behavior could be considered as an intrinsic response of intact human dentin to an applied loading. On the contrary, the brittle deformation behavior of bulk dentin samples, which occurs under tension, is a macroscopic effect caused by a deformation scheme and such a feature of the structure of dentin on the microscopic level as dentin tubulars.