The influence of melt-compounding technique on blast furnace slags (BFS) filled polystyrene (PS) compounds is almost lacking in literature. BFS are by-products of iron industry, and are formed during the production of iron via thermo-chemical reduction in blast furnaces. BFS are mineral structured materials composed principally of different oxides such as silicon oxide (SiO2), calcium oxide (CaO), magnesium oxide (MgO) and alumina (Al2O3) as well as other minor oxides and elements. Such combination of oxides might be of technical advantage if BFS is properly prepared and tailored for use as a functional filler for PS. In addition, BFS is outstandingly inexpensive compared to common mineral fillers used in polymer industry such as calcium carbonate and talc, giving BFS an economic significance. In current study, compounds were produced via melt-compounding approach, where two different processing equipment were used: (1) simple laboratory internal mixer (IM) and (2) co-rotating twin-screw compounding extruder (TSC). Three PS formulations filled with two types of BFS, granulated (G-type) and air-cooled (A-type) were produced. Fillers were incorporated into PS matrix at 20 wt.-% loading and 0-71 and 0-40 µm ranges. Compounds processed via each equipment were compression molded and investigated for rheological, thermal and mechanical performance. It was found that compounding process did not have a strong influence on either rheological or mechanical behavior. However, an influence on thermal behavior was noticed, where transition temperatures (Tg) of TSC compounds shifted to lower values compared to corresponding IM-ones.