Prior studies have shown that poly(3-methylthiophene)-modified platinum electrodes (P3MTPt) have improved electrochemical reversibility, selectivity, and sensitivity for the detection of catecholamines when compared with conventional electrodes. Difficulties have, however, been encountered, as a P3MT film grows beyond the edges of the Pt electrode substrate during electropolymerization, and, consequently, increases the surface area of the electrode. The width of the film growth beyond the edges of the substrate depends on electropolymerization conditions and the size of the electrode substrate. This makes the catecholamine analysis irreproducible from electrode to electrode, and especially limits the application of the P3MTPt system as micro- and ultramicro-electrodes. In order to control the edge effect, optimization of the preparation conditions of P3MTPt electrodes, including temperature, monomer concentration, deposition time, voltage, and electrolyte concentration, was studied by fractional factorial design (FFD). Improved P3MTPt microelectrodes were achieved by applying the optimal conditions, which simultaneously reduced the edge effect. Also, P3MTPt film electrodes polymerized at -20 degrees C exhibited the same electrocatalytic properties, which are essential for detection of catecholamines, as those electrodes made at room temperature.