Sumary: The droplet breakup theory has been investigated in predicting processing conditions of polymer blends. A ternary blend of polystyrene/styrene-butadiene rubber/natural rubber (PS/SBR/NR) was used with a constant content of every component [PS/SBR/NR 80/(30/70) wt.%] while changing mixing temperature, time and rotor speed. The experimental design was applied to optimize mixing conditions where izod impact and tensile strength were the targets. The mixing conditions from theory calculation: 190 °C, 56.5 rpm, 3.47 ÷ 5.47 min. Suitable blending conditions resulting in the experiments were: 190 °C, 60 rpm, 5.44 min.