The performances of an electrochemical detector (ECD) using Prussian blue (PB) modified indium tin oxide (ITO) electrode were investigated. PB is one of the well-known catalytic materials. PB offers electrocatalytic reaction between electrode and analytes (dopamine and catechol). We optimized the PB film thickness that would catalyze redox reaction efficiently using surface topography from atomic force microscopy (AFM). Capillary electrophoresis (CE) and amperometric method were used to separate and detect the testing analytes. The results showed that the PB film could serve as an excellent mediator, which catalyzes electron transfer. We measured electropherograms for the testing analytes consisting of catechol and dopamine with their concentrations of 100 µM and 10 µM, respectively.