Effect of silicon dioxide grain size on the rheological properties of Acrylonitrile Butadiene Rubber (NBR)/Chloroprene Rubber (CR) blends has been studied. Silicon dioxide as a nanopowder filler has been added with (50) wt.% and different grain size particles (0-150 nm), and study the effect of these additives on viscosity, scorch time, cure time and max torque for NBR/CR blends. The results obtained from the rheological tests showed that viscosity and max torque increased with increasing of Silicon dioxide grain size, while the result was inverse with scorch and cure time, where the larger grain size of Silicon dioxide will lead to reduce of scorch and cure time.