This study dealt with operating windows and scale-up rules of a compounding twin screw extruder for polypropylene (PP)/multi-walled carbon nanotubes (MWCNT) composites with required conductivity. In this study we showed that the experimentally obtained boundary line separating either conductive or non-conductive region is essentially linear, and thus the operating windows, in which the PP/MWCNT composite is consistently conductive with statistically 95% reliability level, can be drawn as a triangle. The use of this triangle rule of the operating windows to achieve required volume resistivity was proposed. With similarity of the length and diameter ratio (L/D) and screw configuration between small (laboratory-scale) and large (production-scale)) extruders, our investigation revealed that the most effective scale-up factors for the volume resistivity are (D(large)/D(small))(3) for the throughput, and (D(small)/D(large)) for the screw speed as a new scale-up rule generally valid for any machine size.