Garnet pyroxenite xenoliths from the Hannuoba Cenozoic basalt were formed by magmatic underplating in the uppermost mantle (40～45 km), and represent the crust-mantle transition zone. The electrical conductivity of sintered garnet pyroxenite WD958 was measured at 1.2 GPa and 380～900℃, using a Solartron 1260 Phase-Gain Analyzer. The temperature dependence of electrical conductivity (σ) can be fitted by an Arrhenius equation: σ=σ₀ exp(-ΔH/kT), where T is in Kelvin and k is the Boltzmann constant. Values of the pre-exponential factor (σ₀) and activation enthalpy of electric conductivity (ΔH) of sample WD958 are 97.5 S/m and 1.27 eV, respectively. The water contents of minerals were analyzed using the Fourier transform infrared spectrometry. The average water content in clinopyroxene is 117×^-6 H₂O, whereas olivine is very dry (<1×10^-6H₂O) and the water content in garnet cannot be determined due to alteration. The laboratory-derived electrical conductivity of mantle minerals shows that the calculated conductivity using the Hashin-Shtrikman average can match the measured values by assuming a mixture of hydrogen-bearing clinopyroxene, dry garnet and dry olivine. This demonstrates the contribution of both small polaron conduction and proton conduction mechanisms to the bulk conductivity of garnet pyroxenite, and the sample can be regarded as a resistive matrix with non-interconnected conductive inclusions. If the water partition equilibrium between minerals is preserved at the in situ depth (40～45 km), the electrical conductivity of garnet pyroxenite will be enhanced by 1 order magnitude and the proton conduction mechanism becomes predominant. For the lithosphere with a high geothermal gradient, the temperature at the Moho depth could reach 1 000 ℃ and garnet pyroxenite is characterized by high conductivity. In contrast, under normal geothermal gradients, garnet pyroxenite shows conductivity as low as spinel lherzolite. During magmatic underplating, therefore, the electrical crust-mantle boundary will vary with temperature and water concentration.