Abstract: In testing of the resistance of an insulator (resistivity higher than 10⁹ Ω·m), the electrode added to it and the contact pressure and contact area between the electrode and the measured material are the key factors that affect the test accuracy. In this paper, the pressure-free sintering technique is used to prepare alumina ceramics containing certain manganese oxide, chromium oxide and silicon oxide components, and a self-made graphite sheet electrode is used to measure its resistance with an insulation resistance tester. The study found that the contact resistance with graphite electrodes is lower than that with silver electrodes. It was also found that the measured resistivity is basically stable, that is, the contact resistance is minimised, when the contact pressure between the graphite electrode and the alumina ceramic is not less than 19 N and the contact area is not less than 63.7 % of the material itself. It is concluded that silver electrodes can be replaced by graphite electrodes for accurate and easy testing of high resistance.