Space charge accumulation behaviors at voltage polarity reversal in cross-linked polyethylene (XLPE) and in improved XLPE for dc cable are observed using pulsed electro-acoustic (PEA) method. A huge amount of so called a packet like charge, which was enough to enhance the electric field significantly, is generated in XLPE at the polarity reversal, while it is not observed in the improved XLPE. It enhanced the electric stress locally in the sample by almost twice of the applied average stress. Since the packet-like charge was not observed in the improved XLPE at the polarity reversal, it is applicable to high voltage direct current (HVDC) network using line commutate converter (LCC) system, in which the voltage polarity reversal is necessary to inverse the current flow.