1. Introduction: Titanium alloys are very reactive materials and react instantly with oxygen, forming an oxide film, which gives them excellent anti-corrosion properties [??]. Therefore, titanium alloys have proven to be the most tissue-compatible inorganic materials, especially for dentistry [???]. Nickel-titanium wires, which have a nearly equiatomic composition, were introduced into orthodontic clinical use by Andersen and colleagues in the early 1970s. [???]. They are very popular in dentistry because they have a much lower elastic modulus and wider elastic range than other orthodontic wires such as those composed of beta-titanium, stainless steel and cobalt-chromium-nickel [???]. NiTi alloys have excellent biocompatibility with living tissue and corrosion resistance in various simulated body fluids [???]. These properties originating from the thin spontaneously formed oxide passivation layer consist mainly of TiO2 [???]. For patients with fixed orthodontic appliances, mouth rinsing with fluoride-containing products is an effective method for preventing tooth decay because such appliances have complicated morphologies [???]. However, reduced corrosion resistance of pure titanium and titanium alloys has been reported in fluoride-containing environments which attack the protective surface oxide layer and cause allergic reactions to NiTi wires after insertion [????]. The corrosion resistance of pure titanium and titanium alloys depends largely on fluoride concentrations [???]. Some studies have been reported regarding the corrosion behavior of titanium alloys in fluorine-containing environments [????]. It is claimed that the fluoride-containing solution with fluoride ions above 20 ppm can destroy the protective oxide the...... half of the paper ...... the active layer increases.5. Conclusions The corrosion resistance of NiTi was studied in Ringer solution for different concentrations of chloride ions. Our results show that increasing the concentration of chloride ions can accelerate the corrosion of NiTi alloy. OCP values ​​in contained KCl and NaCl solutions are less negative at lower concentrations than at higher ones, and corrosion resistance decreases as the chloride concentration increases. According to EIS analysis, NiTi exhibits good corrosion resistance even at higher concentrations of chloride ions, but the polarization resistance and capacitance value of the protective oxide layer decrease as the ion concentration increases. Acknowledgments The author acknowledges Shiraz University, School of Materials Science and Engineering for financial supports. (Grant no. 89-GR-Ing-58)