Preparing polymer blends is an effective way to tailor the good properties of plastics but the most commonly used polymers are incompatible with each other. Therefore, to reduce the interfacial tension and to achieve finer and stable morphology, a suitable copolymer or compatibilizer has to be added to blends in order to establish new interactions between the phases. However, it is difficult to determine the required amount of compatibilizers in polymer blends. As an outcome of the present research a novel separation method was developed, where the blends are investigated in melted state, utilising centrifugal force to determine the adequacy of compatibilizers. The effectiveness of styrene/ethylene/butylene/styrene block copolymer grafted with maleic anhydride (SEBS-g-MA) was verified by blending two immiscible plastics: polystyrene (PS) and high density polyethylene (HDPE). FTIR measurements were carried out to support the results of optical microscopy regarding the purity of separation. Comparing the results of morphology, rheology and mechanical properties with the novel separation method, it seems that investigation of compatibilization effect in a melted state would be suitable for predicting the adequacy of compatibilizer in blend. The minimum required amount of compatibilizer was also detectable, wherein the stress-strain curves begins to change significantly and the impact properties starting to improve in PS/HDPE blends.