Blending of polymer is an important technique to obtain a desirable combination of properties which are often absent in pure polymers. Since this procedure is simple, versatile and economical when compared to the synthesis process, it can be used to develop novel polymer blends that meet the requirements of the different polymer processing, such as extrusion, thermoforming, injection and blow molding. In addition, mechanical properties of final product can be improved with this technique. The aim of this work is to evaluate the effect of blends composition of quaternary blends of polystyrene (PS) and high impact polystyrene (HIPS) on the rheological and mechanical properties. The blend compositions were designed using Simplex-centroid Mixture Design using two commercial grades PS and two commercial grades of HIPS. The experiments are carried out in a co-rotating twin-screw extruder and mechanical properties specimens are produced by injection molding. Rheological properties were investigated by oscillatory flow testing using AR2000 Rheometer (TA Instruments). Furthermore, the tensile versus strain behavior and Izod pendulum impact resistances are also carried out. As results, models were developed for describing the relationship between the proportions of each individual polymer in the mixture on the properties studied.