An ultrafast electrophoresis method was developed to have an insight into the interaction of E. coli, DH5 alpha, with an antibiotic gentamycin. The method used micro-video to visually measure the migration of bacteria, rather than molecules, within several micrometers, taking a time down to 0.04 s. Precise bacterial mobility was measured at the electric field strength of 42.5-62.5 V/cm, with a mobility deviation of less than 1.7% compared with a common CE method. The bacterial mobility measured this way was found to have a distribution increasing with the electric field strength applied, but the averaged mobility kept constant or independent of the electric field strength. As indicated by the bacterial mobility measured, DH5 alpha experienced four stages during interaction with the antibiotic gentamycin, namely, the outer surface adsorption equilibration, uptake or transferring of adsorbed gentamycin through cellular membrane, destruction of the membrane, and death of the bacteria. The bacterial outer surface adsorption of gentamycin was found to have duration up to about 5 min that decreased reversely with the concentration of gentamycin added. This proves that the activity of antibiotic gentamycin is dose-dependent.