The principle of an on-line preconcentration method,for capillary zone electrophoresis (CZE) named electrokinetic supercharging (EKS), is described and based on computer 2 simulation the preconcentration behavior of the method is discussed. EKS is an electrokinetic injection method with transient isotachophoretic process, is a powerful preconcentration technique for the analysis of dilute samples. After filling the separation capillary with supporting electrolyte, an appropriate amount of a leading electrolyte was filled and the electrokinetic injection was started. After a while, terminating electrolyte was filled subsequently and migration current was applied. This procedure enabled the introduction of a large amount of sample components from a dilute sample without deteriorating separation. Computer simulation of the electrokinetic injection revealed that EKS was effective for the preconcentration of analytes with wide mobility ranges by proper choice of transient isotachophoresis (ITP) system and electroosmotic flow (EOF) should be suppressed to increase injectable amount of analytes under constant voltage mode. A test mixture of rare-earth chlorides was used to demonstrate the uses of EKS-CZE. When a 100 muL sample was used, the low limit of detectable concentration was 0.3 mug/L (1.8 nm for Er), which was comparable or even better than that of ion chromatography and inductively coupled plasma-atomic emission spectrometry (ICP-AES).