The sintering behavior of 7.8 mol% Yttria-stabilized cubic stabilized zirconia (Y-CSZ) powders was investigated, with emphasis on the effect of Al₂O₃ addition at the initial sintering stage. The shrinkage of powder compact was measured under constant rates of heating (CRH). The powder compact with 0.25 mass% Al₂O₃ enhanced the densification rate with elevating temperature. The activation energies at the initial stage of sintering were determined by analyzing the densification curves. The activation energy of powder compact with Al₂O₃ was lower than that of powder compact without Al₂O₃. The diffusion mechanisms at the initial sintering stage were determined by the new analytical method applied to the CRH data. This analysis exhibited that the diffusion mechanism changed from grain boundary to volume diffusions (VD) by Al₂O₃ addition. It is therefore concluded that the Al₂O₃ addition enhances the densification rate because of decreasing the activation energy of VD at the initial sintering stage.