The current work reports the effect of particle size on the rheological behavior of polymer modified asphalt PMA. The modified asphalt was generated with AC-20 asphalt precursor provided by Pemex Salamanca and Solprene® 416 provided by Dynasol Mexico. Solprene® 416 is a SBS star-type copolymer with 4 poly(b-styrene-b-butadiene) arms and Mw = 2.36X105 g/gmole. Modified asphalt samples were prepared with 3 wt % of SBS via hot mix process. Mixing time and temperature were kept constant at 4h and 180 °C. This study also varied the agitation in the mixing process: 500, 1000 and 1500 rpm. All PMAs shown sphere-shaped polymer particles as observed via fluorescent microscopy using a Carl Zeiss KS-300 system. Base asphalt and PMAs were also characterized through rheological measurements using a TA Instruments AR-G2 rheometer. Shear viscosity (η) and tan δ data shown that the flow resistance of the PMA increases as the size its polymer particles decreases. Since the size of the polymer particles decreases with the increase of the stirring speed, it is concluded that the stirring speed of the process determines the size of the polymer particles and so the mechanical resistance of the PMA.