Electrophoresis characteristics of several monophthalates in the sample zone and EOF variation in a fused-silica capillary column during a constant pressure-assisted electrokinetic injection (PAEKI) in an on-line CE-MS were studied in an effort to reconcile the mobility theory and field amplification with the enhancement achieved in present work. Influences of capillary length on the amount injected using PAEKI were investigated in detail and except for the injection time, the amount injected was found to increase linearly with capillary column length. A longer capillary provides a longer linear increase time range with PAEKI injection. The results show that smaller m/z analytes generate a large enhancement power using PAEKI, which is in agreement with the mobility theory. ACN was used as an example to investigate influences of organic additives on the amount injected and was found to decrease the amount injected with PAEKI injection, which is in agreement with an increase of resistivity in running buffer by organic additives. The peak width obtained with PAEKI injection proved to be independent of the amount injected. The band size of the sample zone was estimated by comparison with conventional hydrodynamic injection. A 240 s PAEKI injection achieved the same size of sample zone as a 2 s of hydrodynamic injection. Existance of two ion layers around the boundary of the buffer and sample solutions in sample zone was hypothesized to contribute the narrow sample zone with a long time of PAEKI injection. With a 240 s on-line PAEKI injection in CZE-MS, five monophthalates were enriched several hundred times without compromise in their separation efficiency and peak shape. With appropriate sample cleanup, PAEKI was applied to the analysis of monophthalates in urine samples, achieving detection limits ranging between 0.53 and 1.3 ng/mL.