The new concept of dissymmetry modeling of cross-section geometries for fully wiped co-rotating screws was proposed.The corresponding mathematical model was established with the fluid concerned obeying the Carreau flow model.Based on the mesh superposition technique，the two-dimensional periodical flow and mixing in co-rotating screws with two kinds of cross-section geometries were simulated with finite element package Polyflow.The distributive mixing was investigated in terms of particle tracking visualization combined with segregation scales.In contrast，the dispersive mixing was characterized by the mixing index together with the shear rate distribution in the flow field.When the sum of rotations of the left screw and the right screw was kept as constant，the group with speed ratio 2∶3 was better in the ability of mixing than the group with speed ratio 2∶1.