The present study investigated the effects of the mixture of multiwalled carbon nanotubes (MWCNT) and carbon black (CB) on the electrical properties of nanocomposites of PMMA polymerized in situ. The level of dispersion and the electrical percolation threshold were considered. The electrical conductivity was measured by the four point probe and the dispersion was analyzed by small angle x-ray scattering (SAXS) and transmission electron microscopy (MET). The dispersion characteristics of MWCNT were superior to CB. It has been shown that the addition of MWCNT and CB promoted the electrical conductivity of the composite as a percolation critical concentration is reached. The percolation threshold results were 0.2 vol. (%) for MWCNT and 1.5 vol. (%) for CB, showing that the difference for the two fillers was associated with their geometric shapes and aspect ratios as well as with the different dispersion characteristics. In addition, blends of MWCNT and CB, varying the concentration of each filler were also prepared. For these blends it was applied the Sun model, which was developed based on the excluded volume theory to predict the percolation threshold. The calculated values were compared to the experimental data and it was observed the synergic effect regarding to the electrical percolation threshold. The blends showed lower critical concentrations (considering the sum of each single filler concentration, MWCNT and CB) and similar conductivities as compared to the single fillers. SAXS showed that the MWCNT, CB and blends form aggregates with fractal structures. The dispersion of both fillers down to the elemental level of a single particle was not achieved. Despite the presence of aggregates the microscopy analysis showed that both particles were well dispersed in the polymer matrix. In the blends the MWCNT act as filaments linking clusters of the CB, what can explain the synergic effect observed by the lower percolation threshold.