The synergetic influence of silane-grafting and polar additives (EVA) on the water tree resistance of the low density polyethylene has been investigated. A series of samples obtained before and after hydration have been characterized by measuring gel content, infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and dielectric measurements. The results obtained clearly show that the silane condensation occurred and that the silane-grafting and polar additives have synergetic effects on the water tree resistance of LDPE with little influence on its dielectric properties, e.g. the dielectric breakdown strength, dielectric permittivity and loss tangent.