The structure features of primary tooth hard tissues were studied at teenage, person of young age and patient's age from 20 to 50 years by means of methods using in physical material science (metallography, x-ray analysis and transmission electron microscopy). The findings, which agree with the results of dental volume tomography, allow determining the features of microstructure of crown and root dentin of patients at different age. On the basis of the this data the criteria of degree of formation and maturity of the root was objectified that allow variably change the endodontic treatment