The sinterabilities of fine zirconia powders including 5 mass% Y2O3 were investigated, with emphasis on the effect of Al2O3 at the initial sintering stage. The shrinkage of powder compact was measured under constant rates of heating (CRH). The powder compact including a small amount of Al2O3 increased 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 including Al2O3 was lower than that of a powder compact without Al2O3. The diffusion mechanisms at the initial sintering stage were determined using the new analytical equation applied for CRH techniques. This analysis exhibited that Al2O3 included in a powder compact changed the diffusion mechanism from grain boundary to volume diffusions (VD). Therefore, it is concluded that the effect of Al2O3 enhanced the densification rate because of decrease in the activation energy of VD at the initial sintering stage.