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To identify the location of partial discharge occurring in a power cable, it is essential to obtain the intrinsic values of permittivity of cross-linked polyethylene (XLPE) material. This paper reveals that when the conventional two-parallel-electrode method is used, and due to the fringing effect, the measured permittivity of XLPE material pronouncedly differs from its intrinsic value, consequently significant measurement error could occur. An effective method based on the field-circuit coupled theory is presented in this paper to investigate the difference between the measured and intrinsic permittivity of XLPE insulation material. A capacitor constructed from a concentrically cylindrical XLPE sample is used. By assuming a certain value of the intrinsic permittivity for XLPE material, the electric field distribution in the XLPE material and the ambient is calculated. Then the total energy is determined, as well as the measurable permittivity. By assuming different values of intrinsic permittivity, corresponding measurable values of permittivity are determined, and a curve of measurable permittivity against its intrinsic value is then obtained, from which the intrinsic value can be found once its measurable value is obtained from experiment. The influence factors of the correction curve are analyzed, i.e. the sizes of sample and electrode. A measurement was carried out to verify this method.