A study on the effect of cation species on the viscosity and electrical conductivity of CaOMgO- SiO2 system is carried out. Rotating cylindrical and two-plate method is used for viscosity and electrical conductivity measurements, respectively. Raman spectroscopy is also carried out to understand the structure of the slags. Experimental results indicated that the cationic effect on viscosity and ionic conductivity is governed by classical Anderson-Stuart theory: The dominance of electrostatic interaction on steric hindrance is confirmed for depolymerized melts (NBO/T=2.0). For polymerized melts (NBO/T=0), however, the major cationic effect on transport properties are examined to be a strain field distortion energy. The viscosity and ionic electrical conductivity are in strong correlation and assumed as the structure-dependent property. The structure of polymerized melts is also affected by the transition in primary solidification phase: Abnormal changes in properties and structure are observed at diopside congruent composition. Such a change at the congruent composition is assured by the entropy calculation and the stability function, ψ.