Boron nitride nanotubes (BNNTs) can be a promising additive in polymer nanocomposites due to their unique mechanical, thermal and electrical properties. In this study, Polystyrene (PS)-Boron Nitride Nanotube (BNNT) composites were obtained through preparation of masterbatch using in-situ polymerization and then processing in an extruder. In-situ polymerization is an efficient method to prepare polymer nanocomposites because this technique can provide well dispersion of nanoparticles starting from the monomer level interactions. Synthesis parameters of the bulk polymerization were initiator concentration, reaction temperature and time which were assessed based on the molecular weight of synthesized PS. The optimum PS synthesis conditions were obtained at a reaction time of 3 hours, an initiator concentration of 0.01 mol/L and a reaction temperature of 95 C. The composite preparation was started with PS masterbatch synthesis at high BNNT inclusion then followed by melt blending with PS to obtain polymer composites at 0.5, 1 and 3 wt% of BNNT loadings. PS/BNNT composites were characterized in terms of their morphology, thermal and mechanical properties. In 0.5 wt% BNNT loaded composite the nanotubes were observed to be more homogeneously dispersed rather than the other BNNT loadings at 1 and 3 wt%. Agglomeration of BNNTs in the polymer matrix increased with the increase in BNNT addition. More dispersed nanotubes at 0.5 wt% BNNT loading led to higher tensile and impact strengths with respect to the neat PS and the composites with 1 and 3 wt% BNNT loadings.