The x values and electrical conductivities of the nonstoichiometric compounds$ CeO_{2-x} have been measured in a temperature range from 600 to 1200$^{circ}C$ under oxygen partial pressure of $2{ imes}10^{-1}{sim}1{ imes}10^{-4}$ atm. The enthalpy of the defect formation shows an endothermic process with the oxygen partial pressure dependence (1/n value) of -1/3.18 ∼ -1/3.69. The activation energy and 1/n value for the electrical conductivity are estimated as 1.75 eV and -1/4, respectively. According to the x values, the $sigma$ values, and the thermodynamic data, the defect structure of the ceria seems to be the formation of singly charged negative oxygen vacancies. The n-type semiconducting behaviors could be explained by the presence of excess metals in the lattice as the conduction electron donor.