Smart orthodontic alloys are used in orthodontic practice to correct the position of teeth, especially in children. For the choice of orthodontic arcs, the dentist must know some of their physical characteristics: the force that develops under stress conditions, the elasticity range and the thermal characteristics of the alloy, including the phase transitions. The main orthodontic arcs are made of gold, Co-Cr-Ni alloys, stainless steel or Ni-Ti alloys. The Ni-Ti alloy, called nitinol, has proven its orthodontic efficiency, especially due to its remarkable elasticity and shape memory effect, due to the transition between the martensite and austenite phases. The shape memory effect is the return to the initial form after a plastic deformation that occurs at a low temperature [1]. In this paper we study phase transitions of orthodontic wires with different geometries in the transversal section: circular, square, rectangular. Figure is a diagram of the phase transitions of a 0.19x0.25 inch orthodontic wire with two heating-cooling cycles in air at a heating rate of 2 K / min in the temperature range RT-150 oC. The martensite-austenite phase transition occurs in the temperature range of 25-38 oC, temperatures obtained at introduction into the oral cavity. In addition to the thermal properties, in the work, other physical properties of the orthodontic alloys are studied: magnetic, thermomechanical, structural and adsorption properties.