NOx emission from the ternary eutectic molten salt (7 wt%NaNO3-40 wt%NaNO2-53 wt%KNO3, HITEC) has been investigated in thermal energy storage (TES) processes. The effect of different metal and exposure time on NOx emission is evaluated, including Incoloy 625 and Hastelloy C276 degrees of nickel-based alloys, and AISI 310S (310S) and AISI 321 (321) of stainless steels. The mechanism of NOx emissions is analyzed by using thermodynamic calculation, scanning electron microscopy (SEM), energy dispersive X-Ray (EDX), X-ray diffractometer (XRD) and atomic absorption spectrophotometer (AAS). The experimental results indicate that HITEC can release less than 40 mg/m(3) of NOx when exposing to In625 or HasC276 alloys below 500 degrees C, and more than 350 mg/m(3) of NOx for 310S and 321 at 500 degrees C. It is found that oxide film is formed on metals surface for long exposure time in HITEC resulting in the decrease of NOx emission. The calculated Gibbs free energy change reveals that above 500 degrees C, Cr2O3 protective films on the metals are reduced, which contributes to rapid increase of NO2 and NO emissions. In625 or HasC276 alloys is recommend to be used in pipe line/container, and the NOx emissions of HITEC based TES system can be effectively decreased especially for long-term energy storage.