For molten (Na, K, Cs)CI system, the relative differences in the internal cation mobilities at 1023 K are investigated by the Klemm method and the electric conductivity by a direct current method. From these data and the molar volumes which are evaluated from those of the pure salts on the assumption of additivity, the internal mobilities of the each cation, ie., b(Na), b(K), and b(Cs) have been calculated. This result allows us to conclude that the internal mobilities of each cation become b(Cs) < b(Na) < b(K) at x(Cs) = 0.1, b(Na) < b(Cs) < b(K) at x(Cs) = 0.14 and b(Na) < b(K) < b(Cs) at x(Cs) > 0.6. In addition, the molecular dynamics simulation has been performed on molten (Na, K, Cs)CL for the various compositions at 1023 K. Then, the self-exchange Velocities (SEVs) of Na+, K+, and Cs+ with reference to Cl- have been calculated. The sequence of the internal mobilities of these cations is well reproduced by the corresponding SEVs.