This paper was focused on thermal stability of molten salts and their thermo-physical properties at high temperature. In this experiment, multi-component molten salts composed of potassium nitrate, sodium nitrite and sodium nitrate with 5% additive A of the chlorides were prepared by statical mixing method. The experiments found molten salt with 5% additive A had higher thermal stability and its best operating temperature would be increased to 550 degrees C from 500 degrees C when comparing to ternary nitrate salt. Meanwhile, thermal stability and thermal cycling analysis showed molten salts with 5% additive A had lower freezing point and loss of nitrite content and deterioration time of molten salts were reduced at the same time. DSC tests also indicated loss of latent heat in molten salts with 5% additive A was decreased. Besides, thermo-physical properties measured showed molten salt with 5% additive A had a heat capacity of 2.32 kJ/kg degrees C, lower than 4.19 kJ/kg degrees C for water between 0 degrees C and 100 degrees C and a low viscosity range from 3.0 to 1.4 cp between 150 degrees C and 500 degrees C, analogous with 1.8-0.3 cp for water between 0 degrees C and 100 C. Other thermo-physical properties, such as thermal conductivity, density and linear thermal expansion, were also determined here. (C) 2009 Elsevier Ltd. All rights reserved.