The estimation of the functions that describe the shear stress and the pressure drop across the short orifice die as a function of shear rate at different mean pressures for linear-low-density polyethylene copolymer (LLDPE) at 190oC is obtained using Genetic Programming (GP) . The GP has been running based on experimental data in two cases; shear stress and pressure drop at different mean pressures to produce shear stress and pressure drop for each target mean pressure. The shear rate and mean pressure of linear-low-density polyethylene copolymer have been used as input variables to find the discovered functions. The experimental, calculated and predicted shear stress and pressure drop are compared. The discovered function shows a good match to the experimental data. We find that the GP technique is a good new mechanism of determination of the shear stress and the pressure drop linear-low-density polyethylene copolymer.