Effects of degree of crosslinking in butyl rubber on the flow behavior and morphology of crosslinkable polyethylene (XLPE) and butyl rubber blends have been studied as a function of shear rate and processing temperature. Viscosity, flow activation energy, and rheological parameters have been studied with reference to the effect of blend compositions. Low-crosslinked butyl rubber showed higher normal stress difference than the highly crosslinked butyl rubber in the blend. As evidenced from extrudate swell, melt compliance, and Reynolds number, phase reversion in low-crosslinked butyl rubber occurred within a narrow composition range whereas in highly crosslinked butyl rubber blends, the change occurred over a wide composition range. Change of phase has also been confirmed by SEM examination. Highly crosslinekd butyl rubber showed higher elastic response than low-crosslinked rubber.