Morphology development was investigated during blending of linear low-density polyethylene (LLDPE) and polystyrene (PS) (the minor phase) in a Haake internal mixer. In addition, the blending was studied using 5 wt % Kraton styrene-ethylene/propylene (SEP) diblock or styrene-ethylene/butylene-styrene (SEES) triblock copolymers as compatibilizing agents. Scanning electron microscopy (SEM) examinations of blends show a bimodal particle size distribution of the minor phase during the initial stage of morphological development. Morphology observations suggest that the main function of the compatibilizer during blending lies in reducing the interfacial tension between two immiscible polymer phases. The addition of the copolymer reduces the average domain size from a micron to a submicron range. Compatibilization mechanisms of the copolymers with the immiscible blends are proposed. The superiority of SEES triblock over SEP diblock as a stabilizer of the morphology was shown.