In stabilized-ZrO2 with high-purity, the space-charge contribution plays a dominating role in the grain-boundary resistance. A grain-boundary conduction model is proposed on the basis of this concept, and the effect of the segregations of calcium, yttrium and aluminium on the space-charge conduction is discussed, the segregations are all found to decrease the space-charge conduction, thus increase the grain-boundary resistance. Some formerly-established concepts are evaluated via the model, the limitation of the complex impedance approach based on the Bauerle′s equivalent circuit is pointed out, and the defect of the concept of the resistance per unit interface area of the grain boundaries is also discussed.