Three kinds of dispersing agents, all copolyesters, were synthesized from the polycondensation reaction of dimethyl terephthalate (DMT), dimethyl isophthalate (DMI), sebacic acid (SA), and 1,4-butandiol (BD). Copolyester from DMT, SA, and ED was designated as PBTS; from DMT, SA, DMI, and ED as PBTIS; and from DMT, DMI, and ED as PBTI. Carbon black masterbatches were prepared by mixing carbon black into the dispersing agents in a Brabender Plasticorder PLE331. Using single screw extruder, masterbatches were compounded with poly(ethylene terephthalate) in 3 wt% concentration and mechanical properties of the compounds were investigated. Gel permeation chromatography data implied that thermal degradation of polymeric dispersing agents was not very significant. Capillary rheometer tests showed that PBTI has the highest viscosity and shear sensitivity of the three dispersing agents. Volume resistivities of each masterbatch were increased as the rotor rpm of the Brabender and mixing time increased. Transmission electron micrographs showed that dispersion of carbon black was enhanced by increasing the melt viscosity of the dispersing agent. The performance and mechanical characteristics of carbon-black-filled PET compounds depends directly on the dispersion quality of the carbon black in the masterbatch. Mechanical properties of compounds were improved by increasing dispersion of carbon black and increasing numbers of rigid aromatic groups in the copolyester dispersing agent.