In order to evaluate the performance of different screws designs, decrosslinking of the crosslinked high density polyethylene (XHDPE) is performed by means of an ultrasonic twin-screw extruder (TSE) with two screw configurations. Die pressure and ultrasonic power consumption during extrusion of XHDPE is recorded. Swelling test, scanning electron microscopy (SEM) and tensile test are used to investigate the structure and properties of the decrosslinked XHDPE. It is found that the screw configuration without kneading elements (decrosslinking screws) is more effective in decreasing the gel fraction and crosslink density of the decrosslinked XHDPE. The ultrasonic treatment significantly improves the processibilty of the decrosslinked XHDPE. The reason for the superior performance of this screw configuration for decrosslinking of XHDPE is explained by extrusion theory. The numerical calculation describes the effect of the residence time and the gel fraction and crosslink density of the initial polymer on the gel fraction and crosslink density of ultrasonically decrosslinked XHDPE. The mechanical performance of the decrosslinked XHDPE from TSE containing the decrosslinking screws is very close to those of the virgin XHDPE. The effect of the gel fraction and crosslink density on the morphology of the decrosslinked XHDPE is also identified.