Studying the morphology and mineralization of the crown and root dentin in children’s first permanent premolars, removed according to the orthodontic indications at the age of 13-14 and 16-18 years old, was carried out using the transmission electron microscopy method. It was found out that the crown and root dentin in the first permanent intact premolars in children of 13-14 years old is in the amorphous state. At the age of 16-18 years, presence of collagen fibres in the dentin and formation of nanocrystalline phase of calcium hydroxyapatite in the dentin matrix were determined. At the age of 13-14 years, a slight presence of phosphorus in the root dentin of premolars is found out. The Ca concentration in the crown and root dentin of the first premolars is practically in the equal ratio. The structure of the dentin (crown and root) in the children’s permanent teeth goes through a long stage of formation and mineralization.