This work aims at the simulation of three-dimensional isothermal flow between a pair of discs in series disc-screw (SDS) extruder by computational fluid dynamics. To analyze the distributive mixing performance and overall efficiency of discs, mixing parameters such as segregation scale and time-average efficiency were calculated. The effects of the disc configuration and the distance between a pair of discs on the distributive mixing were discussed. The more grooves on the disc, the better distributive performance. Good distributive mixing performance can be obtained by big gaps.