SUMMARYWater treeing is one of the main deterioration phenomena affecting the service lifetime of the underground crosslinked polyethylene (XLPE) power cables. Factors influencing the water tree growth can be classified in two major groups, namely, the external factors (operational factors) and the internal factors (material factors). The influences of the external factors on the number and the growth rates of water trees have fully understood through the long-term research around the world for the past four decades. However, the effects of the internal factors have been not so much investigated as those of the external ones. Especially, it has been hardly discussed which among the probable internal factors is the most influential to the water treeing phenomena until now. In this paper, the authors have investigated the contributions of individual factors influential to the electric field distribution in the tip of the water tree initiated from the outer surface of XLPE cable insulation considering its thermal degradation, using both the finite element method (FEM) and Taguchi design of experiment methodology (Taguchi method). The numerical and water treeing experiments show that the dielectric properties of the water tree and the XLPE host material are more influential to the enhancement of the electric field intensity in the tree tip than its geometrical parameters, and that the permittivity distribution inside the tree is the most influential factor among all of the internal factors.