Abstract: Numerical study of two-dimensional isothermal bi-component co-extrusion under wall slip boundary conditions was performed by using POLYFLOW package.The distributions of velocity, pressure, viscosity and shear rate for co-extrusion flow field were solved, with and without the same wall slip coefficients on both wall sides respectively.The effects of wall slip on co-extrusion flow field, interface shape and extrudate swell were discussed. The simulation results revealed that, when wall slip coefficients were kept the same on both wall sides, both interface shift and extrudate swell ratio increased with slip coefficients; when wall slip coefficients were different on both wall sides, interface shift became more significant as the difference of slip coefficients increased, and the melt with a smaller slip coefficient swelled more evidently.