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In order to clarify the mechanism of electrical-tree inception and propagation from a water tree within polyethylene, we have investigated effects of water-tree degradation condition and impulse-voltage application on electrical-tree propagation from water tree. As for the condition of water-tree degradation, we have investigated electrical-tree inception voltage by using NaCl solution with different concentrations. Regardless of concentration, the inception voltage becomes higher with the water-tree size. This is explained by electrical-field moderation. On the other hand, under impulse-voltage application where less field moderation is expected, the increase in tree inception voltage is much higher. This suggests increase in intrinsic breakdown strength in and/or near water-tree region.