In this study, the effect of the molecular weight of the polymer matrix on the dispersion of multi-walled carbon nanotubes (MWNTs) was investigated for high-density polyethylene (HDPE)/MWNT and polycarbonate (PC)/MWNT composite materials. The dispersion of MWNTs in the polymer matrix showed a strong correlation with the melt viscosity of the polymer matrix, i.e., with the molecular weight of the polymer material. Several experimental methods, such as 4-point probe, SEM and ARES, were used to examine the effect of the molecular weight of the polymer matrix on the dispersion of the MWNTs and various physical properties of the composites. The use of a high melt viscosity polymer as the matrix material restricted the mobility of the MWNTs, and also hindered the dispersion of MWNTs due to the high entanglement density of the polymer matrix.