A method of preparing a plastic substrate for electrostatic painting includes preparing a carbon nanotube pellet by molding carbon nanotube powder. The method also includes preparing a conductive resin composition by mixing 0.1 to 10 wt % of the carbon nanotube pellet, 0.1 to 20 wt % of carbon black, and 70 to 99 wt % of a thermoplastic polymer resin. The method further includes molding the conductive resin composition.