Abstract: To investigate the influence of the water tree propagation on the damage of the crystalline regions of cross linked polyethylene (XLPE), this paper investigated the changes of the structural characteristics and damage mechanism of XLPE crystalline region in the early period of water tree propagation. Three groups of XLPE sheet samples A-C were subjected to an accelerated water tree aging for 10 days, 20 days, and 30 days, respectively. Afterwards the crystalline region morphology in water tree regions was observed by scanning electron microscope (SEM) after erosion, and the crystalline structural changes were detected by X ray diffraction (XRD). SEM observation results showed that the defect morphologies in the crystalline regions change in different aging periods. After 10 days of aging, slender screw dislocation slipping lines and short edge dislocations were observed in the samples. After 20 days of aging, microcracks were initiated along the crystal face directions in the samples. After 30 days of aging, microcracks developed into several ten micrometers. In the meantime, sharp cornered dislocation etch pits existed in the samples, and there were microvoids at the bottom of the etch pits. XRD detection results showed that water tree aging could result in the emergence of the sharp cornered distortions in the diffraction peak and the increase of the half-peak widths. In addition, the crystalline degree of the samples decreased with aging. It was analyzed that the crystalline regions of XLPE can be damaged by water tree aging. When the microvoids in amorphous regions developed to the crystalline regions, they exerted electro-mechanical forces on the crystalline regions and they deformrf, further initiating dislocations. Slipping lines and dislocation steps were then formed by the movement and proliferation of the dislocations. With the increase in the dislocation densities, the dislocations in different crystal faced intersect and resulted in the stacking of dislocations. As a result, microcracks and dislocation etch pits were initiated, thus resulting in the damage of the crystalline regions.