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Tree initiation voltages (TIVs) for LDPEs with additives of the polycyclic compounds were measured to investigate the fundamental role of the compound in the suppression of tree initiation. Five types of polycyclic compounds were used as additives: naphthalene (Np), anthracene (An), 9,10-dibromoanthracene (Br-An), tetracene (Te), and pentacene (Pn). The experimental results indicated that TIV depends on the typed of the compound: TIV for LDPE with "An" is about 3.5 times higher than that for LDPE without an additive; TIVs for "Np", "Te" and "Pn" are not significantly increased. The mechanism for the dependence of TIV on the type of the compound was discussed based on the following experimental investigations: the fundamental electric characteristics for the test materials (conduction current, permittivity and tan delta); the spectra for UV absorption of the test materials due to the excitation of the compound and the spectra for the luminescence emitted from the test materials due to the deexcitation; the shape of the precipitates of the additives in LDPE; and the surface feature for the test materials by AFM. The measurements and observations suggested that the highest TIV for LDPE with "An" is accomplished mainly by two factors: the relaxation of the electric field due to the semi-conductive precipitates of plate-shape crystal, and the shallower depth of micro-spaces located at the interface between LDPE and the electrode. Although the excitation of additive increases TIV, it is not a significant factor in the dependence of TIV, because the spectra indicated that the excitation occurs even in the materials with additive, which is ineffective to the increase in TIV.