The use of polymer mixtures filled with conductive nanofillers, i.e. polymer blend nanocomposites (PBN), has increased greatly due to a high demand for advanced conductive materials. The new requirements are based on materials that are lightweight, easy to process, easy to shape, and less expensive than metals so that they can be used in electrical applications (e.g. automotive, electronics, aircraft). A great number of PBN are immiscible, leading to phase-separated structures (e.g. co-continuous morphology), which influences the final properties of the PBN. One of the main challenges when producing PBN is obtaining a homogeneous dispersion of fillers within the polymer, which is required for excellent electrical properties. Among the plethora of conductive nanofillers, high aspect ratio fillers such as multi-walled carbon nanotubes (MWCNT) are the most preferred fillers when the aim is to achieve low electrical percolation threshold and high electrical conductivity in the material. A masterbatch (MB) (polymer incorporated with a high concentration of MWCNT) is often used and the MB is diluted into the pure polymer to prepare the nanocomposite. This technique has been shown to improve dispersion and achieve low electrical percolation threshold in PBN. In this thesis, we control the final morphology in the multiphase polymer materials, particularly we are interested in how to manipulate localization of MWCNT in one of the phases in polyolefin blends (i.e. High density polyethylene/polypropylene HDPE/PP). This is a powerful technique to reduce filler content and tune electromagnetic interference (EMI) shielding properties. Two major studies were conducted to better understand how to successfully design polyolefin blends nanocomposites: 1) effect of order of addition of MWCNT by using different types of masterbatches (MB) (i.e. PE-MB and PP-MB) on the filler localization, final morphology and electrical conductivity; and 2) effect of MWCNT concentration in the MB (20 and 5.6 vol%) on the morphology development and EMI shielding performance. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and optical microscopy (OM) were used to characterize the MWCNT filler localization, phase morphology and MWCNT state of dispersion, respectively. Incorporation of MWCNTs resulted in a modification of the morphology and electrical properties. For instance, MWCNT induced continuity of PE phase, which led to enormous improvement in electrical properties via double percolation. The order of addition of MWCNTs into the blend also had an enormous effect on the blend microstructure. If the MWCNTs were first pre-mixed with HDPE and then diluted in HDPE/PP blend, the tendency was to decrease the domain size of the droplets, regardless of HDPE used. However, when MWCNTs were first pre-mixed with PP, and then diluted into HDPE/PP blends, an increase in domain size due to increased coalescence was observed. Moreover, MWCNTs were located in PE phase, due to higher PE/MWCNT affinity, regardless of whether MWCNTs were pre-mixed first with PP phase or type of masterbatch used (LDPE-5.6 MB or LDPE-20 MB). This result is contrary to predictions using wetting coefficients, which are widely used in the literature. Blends prepared with the less concentrated MB (5.6 vol% MWCNT) showed higher conductivity and EMI shielding due to double percolation and an optimum state of MWCNT dispersion. This indicates that double percolation in blends of polyolefins is a powerful way to enhance electrical properties.