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Resistance to the electrical tree in the insulating material of mixed addition polymer nanocomposite was studied under ac high-voltage application. The polymer nanocomposite was made by the mixed addition of AI2O3 nanoparticle ('Al') and phthalocyanine compound ('Pc') into LDPE. Any coupling agent to avoid the clusters of 'Al' and 'Pc' was not used in the preparation of the test sample. The experimental results indicated that the tree initiation voltages (TIVs) for the 'Pc' and 'Al' mixed addition and the 'Pc' single addition (LDPE with 'Pc' only) were about 1.8 times higher than that for the base polymer. However, TIV for the 'Al' single addition did not significantly increase. The results suggested that the increase in TIV for the mixed addition composite is due to the existence of 'Pc' in the polymer, the mechanism of which was discussed basing on the effects of large π-electron cloud in the molecule of 'Pc' and the effect of semi conductive characteristics of 'Pc' crystallite. On the other hand, the period of time from the tree initiation to the breakdown for the mixed addition was almost 10 times longer than that for the base polymer. In the cases of the single addition, the period of time to the breakdown for the 'Al' single addition was about 3 times longer than that for the base polymer, and the period of time for the 'Pc' single addition was almost the same as that for the base polymer. AFM observation suggested that the well dispersion of 'Al' (nanoparticle) in the polymer is achieved in the mixed addition composite which leads to the wide area of the interface between 'Al' and LDPE. The wide area of the interface was found to control the propagation of tree and accordingly provides the long period of time to breakdown. Consequently, both the increase in TIV and the long period of time to the breakdown were obtained by the mixed addition polymer nanocomposite.