Esthetic orthodontic archwires (fiber reinforced composite archwires) are considered as the latest addition to orthodontic's world, and the load deflection rate is thought to be the most important mechanical property of archwires which has strong effect clinically, and this property is affected by the cross section diameter and the material's type, for these reasons this study aims to investigate the effect of the material's type and the cross section diameter of orthodontic archwires on the load deflection rate of fiber reinforced composite(FRC) archwires with a diameter of 0.014 inch and 0.016 inch which are suitable for the leveling and alignment stage, and to compare these new materials with Nickel titanium(NiTi) archwires .