The water tree retardation effect of EVA (ethylene vinyl acetate) blended XLPE (cross-linked polyethylene) was investigated. In addition to water tree tests such electrical properties as dissipation factor and AC breakdown strength and such mechanical properties as Young's modulus and toughness were also studied, because the performance of insulating materials cannot be estimated only on the basis of water tree retardation. It is shown that, in the case of XLPE blended with EVA, water treeing can be retarded considerably and toughness corresponding to the stress-crack resistance improved with increasing VA content, while the dissipation factor which depends on the concentration of permanent dipoles increases. No marked change of AC breakdown strength with increasing the VA content was observed. It is concluded that the growth of water tree is strongly influenced by the mechanical properties, and improvement of stress-crack resistance may be a possible way to reduce water treeing.