The purpose of measuring viscosity is to define fluid characteristics of molten carbonate electrolyte as it has been widely used as an electrolyte in DCFC system. In DCFC system, several methods to achieve higher efficiency and to produce more electricity have been investigated. One of the methods is to supply fuel with electrolyte in electrolyte circulating system. The important factor for this system is Reynolds number as it can be used for defining fluid characteristic. In order to get the Reynolds number, viscosity has to be defined first. It is not easy to obtain viscosity especially in high temperature liquid. In this study, falling sphere viscometer was used to obtain viscosity at a certain temperature. Mole percentage of binary molten carbonate mixture was also changed to investigate the relation viscosity with mole percentage. The fluid characteristics of molten carbonate were defined by the viscosity obtained by falling sphere viscometer and the mole percentage of binary molten carbonate mixture.