Depending on the morphology of the material and applied voltage frequency, three kinds of electrical trees can exist in cross-Linked polyethylene (XLPE) cable insulation, which are conducting, non-conducting, and mixed trees with different growth mechanisms. It is suggested that when the needle is inserted into large spherulites, conducting trees will form in those spherulites; when it is inserted among spherulites, non-conducting trees will appear along the boundaries of spherulites. Frequency will accelerate the growth of non-conducting trees but have little influence on the initiation and growth processes of conducting trees. If the initiation process of non-conducting trees is too difficult, they will grow into mixed trees. Finally, it is concluded that the space charge limited tiny breakdown around the tips of electrical trees is responsible for the propagation process of conducting trees; on the other hand, fast expansion occurs due to local high temperature and pressure along the boundaries, partial discharge in electrical tree paths and charge recombination, etc., which are the main reason for the growth of nonconducting trees. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 3325-3330, 2009