The dispersion of carbon black (CB) in an immiscible polymer blend of poly(methyl methacrylate) (PMMA) and polypropylene (PP) was studied as the viscosity of PMMA increased. It was found that the dispersion of carbon black was strongly influenced by the viscosity of PMMA. The CB was dispersed in the PMMA phase when the viscosities of PMMA and PP are comparable, as predicted by Sumita's model. As the viscosity of PMMA increases, the CB was found to be located at the interface between the PMMA and PP phases. On further increase in the viscosity of PMMA, the CB was found to be dispersed in the PP phase. In addition, the CB dispersion in the PP/PMMA blends can significantly influence the positive temperature coefficient (PTC) intensity of the blends.