Abstract: Integrated Substrate Gap Waveguide (ISGW) realizes self-package of microstrip lines by using three-layer dielectric substrate, which reduces the radiation loss of circuit. As a multilayer non-uniform dielectric transmission line, the effective relative permittivity of ISGW has not been studied, which seriously restricts research on application circuit of ISGW. To issue this problem, the paper proposes a method of extracting ISGW effective relative permittivity. Firstly, a millimeter wave model of ISGW is established, and the dispersion and transmission characteristics of the model are simulated. Then, a half-wavelength method based on impedance matching is proposed to extract the effective relative permittivity of ISGW, which is derived for the extraction formula of effective relative permittivity. Finally, through the simulation of ISGW model, the effective relative permittivity is extracted from the simulation result S_11 of ISGW model, which is compared the result of effective relative permittivity from the simulation result in software of ANSYS, verifying the validity of the extraction method.