Viscosities of Li2CO3-Na2CO3 and Li2CO3-K2CO3 binary melts, which are assumed as the electrolytes of molten carbonate fuel cell, have been measured in entire composition range. A gas injection system was introduced and the torsion wire replaced with thick one to obtain the reliable viscosity of carbonate melts. lsothermal viscosities showed downward convex curves in entire composition range for both melts. Highest viscosity was found for Li2CO3 and minimum values were found at 70∼80mol% of Na3CO3 and K2CO3 for higher temperatures. The activation energies for both melts decreased monotonously with increasing Na2CO3 and K2CO3, Therefore, it is considered that these melts are simple mixture and the decrease of viscosity is due to volume expansion by mixing. The viscosity at the working temperature of molten carbonate fuel cell, about 923K, is 7∼9mPa·s.