Weld-lines degrade the mechanical and optical properties of products. This is because the polymer molecules near the weld-line highly orient owing to the elongational flow and the molecular orientation is fixed by solidification of the polymer melts before it returns to a random condition. Birefringence experiments were, therefore, carried out for welding flows of a low density polyethylene (LDPE) past a spider supporting a mandrel. The die used in the present study has a glass window to observe the birefringence pattern. The birefringence in the polymer melts was measured by the photoelastic method. The purpose of the present study is to elucidate the effect of the die temperature, spider shapes and a viscoelastic property of polymer melts on the anisotropy of the molecular orientation in the stress relaxation process downstream of the spider. The birefringence pattern in the welding flow of LDPE was measured at the die temperature Td=190, 205°C and the polymer melts temperature Tp=19O°C for flow rates of 0.7, 1.0, 2.2, 2.9, 4.6cm3/s. The results showed that complete relaxation of birefringence depended on the flow rate. For high die temperature (205°C), the distance necessary for the relaxation of molecular orientation was short. Moreover, the birefringence pattern in the welding flow was numerically calculated using the single-mode Giesekus model as a constitutive equation, and the results were compared with the experimental data. The numerical predictions agreed with the experimental results.