This research aims at reducing the color-mark defect in the extrusion blow molding process for plastic gallons, where the mark is a line having different color from the gallon. As per a preliminary study, there were four process parameters affecting the defect appearance; i.e. temperature at the mid-section and end of the extruder barrel, screw speed and hole diameter of breaker plate. After conducting a set of experiments associated with the 2k factorial design, the statistical analysis revealed that the temperature at the middle barrel, temperature at the end of barrel and the hole diameter of breaker plate were the significant factors with regards to the defect. The optimum process condition for making the plastic gallon was found to be 170 ºC at the middle barrel temperature, 190 ºC at the end barrel temperature and 3 mm hole diameter of breaker plate. With the use of this condition, the defective product can be decreased from 0.84 % to 0.02 %.