The present work investigated the breakdown characteristics of high-voltage crosslinked polyethylene (XLPE) cable by electrical trees under ac and composite voltages. The electrical trees resemble either a tree or a bushy structure. The importance of the Weibull parameters for the present study was emphasized. The failure zone of the XLPE cables was characterized by experimental techniques such as wide-angle X-ray diffraction, differential scanning calorimetry, and thermogravimetric-differential thermal analysis, to understand the phase constituents affected by electrical trees. The impact test and flexural test results indicate that material with high stiffness/toughness allows tree formation and causes early failure of the material. The characteristic variation of the aged XLPE cables was investigated by a dynamic mechanical analyzer (DMA). The activation energy values were calculated from the DMA data. The rate of tree propagation was found to be less for materials (XLPE) with high activation, energy. © 2004 Wiley Periodicals, Inc.