Abstract. In this study, the effect of multi walled carbon nanotube (MWCNT) on microstructure development and mechanical performance of MWCNT-filled Polystyrene-b-poly (ethylene-butylene)-b-polystyrene (SEBS) were studied. The nanocomposite samples with different amount of MWCNT loading were prepared by melt compounding using an internal mixer. The result of frequency sweep experiment showed a pronounced low frequency nonterminal storage modulus behavior which was considered as an indication of a 3D physical network formed between nanoparticle-nanoparticle and/or nanoparticle-matrix as a result of good dispersion of MWCNT. The MWCNT induced liquid-solid transition at low frequency range was explained in more details by Cole-Cole plot and Han plot. The result of storage modulus as a function of MWCNT showed a rheological percolation threshold around 1.2%. The addition of MWCNT into SEBS matrix was found to have a significant enhancing effect on modulus and tensile strength with little decrease in elongation at break. For example, the addition of 1% MWCNT enhanced tensile stress and modulus at 300% up to 135% and 60% respectively.