To study the effect of additives on water tree generation, experiments of water tree generation in LDPE and XLPE containing nonionic surfactants were carried out. Among the materials studied some kinds of nonionic surfactant with small HLB value were discovered to have the effect of water tree suppression. In addition to the preliminary experiment of examining the effect of water tree suppression, two kinds of another nonionic surfactant with HLB value smaller than 6.5 were investigated. The result of the experiment demonstrated that the surfactants examined showed a significant decrease of bow-tie and vented type water tree generation. On the basis of the result of the experiments, the mechanism of water tree suppression of surfactants was discussed. It was presumed that the surfactant in hydrophobic polymeric materials form micelles, which could accept the excess moisture in the dielectric material under voltage stress and suppress the generation of water tree.