The temperature and the oxygen partial pressure dependencies of hole and electron conductivity of 20 mol % YO1.5 doped CeO2 (20 YDC) were measured by a dc polarization method using a Hebb-Wagner ion-blocking cell from 973 to 1273 K at oxygen partial pressures log p(O2)[MPa] = -9 to -1. The electron conductivity is proportional to p(O2)(-1/4) in the lower p(O2) region. In the high oxygen potential region, a small polarization method was applied to avoid the failure of gas tightness at the ion blocking interface. This enabled the hole contribution, which is found to be proportional to p(O2)(1/4) in the higher p(O2) region to be distinguished. The activation energy of electron conduction is 2.25 eV (at log p(O2) [MPa] = -7). The activation energy of hole conduction is 1.24 eV (at log p(O2) [MPa] = -1).