Rheological properties and processability of thermoforming were studied for high-density polyethylene (HDPE) and a blend of HDPE with crosslinked HDPE (xHDPE). Blending the xHDPE, which enhances melt strength and strain hardening in elongational viscosity of HDPE, helps the sheet avoid sagging in thermoforming. Moreover, the product of the blend obtained by vacuum forming has uniform wall thickness. Melt strength and strain hardening of the blend were, however, depressed by a processing history in a single-screw extruder, whereas reprocessing by a two-roll mill enhanced the melt strength again. It is considered that the processing history by a single-screw extruder, in which shear-dominant flow takes place, depresses the trapped entanglements between network chain of xHDPE and linear HDPE molecules, and results in low level of melt strength.