Generally, electrokinetic potential has been considered as an intensive property. In other words, electrokinetic potential is not affected by the amount of particulate matter. Montmorillonite, one of essential inorganic matter, was chosen to measure electrokinetic potential. The result of electrokinetic potential measuring experiment showed that the value observed to decrease as the amount of montmorillonite clay increased. This is due to the fact that total ions that adsorbed per unit mass were decreased as the amount of montmorillonite was increased. As a result, electrokinetic potential is considered as an extensive property. By using these results, total interaction energy of suspension was also checked, and revealed that total interaction energy was decreased as the amount of montmorillonite increased.