The numerical approaches we originally developed were applied to the rotor element of a counter-rotating continuous mixer in order to evaluate the mixing performance. Our approaches include the combination of the 3-D FEM based flow analysis and the marker-particle tracking method. The kneading block of a co-rotating twin screw extruder was also investigated in order to contrast the mixing behavior with the rotor. The numerical results showed that the stress induced mixing in rotor elements totally depends upon the process conditions and high rotational speed achieved the uniform mixing. The uniform mixing was expected when the polymer melt underwent narrow stress distribution in the flow channel. The numerical approaches are the effective method to design the process conditions in order to improve the stress induced mixing.