The sintering behavior of fine-grained ZrO₂ (3mol%Y₂O₃) powder compacts containing 10-30vol% Al₂O₃ during constant-heating-rate sintering has been studied, with particular emphasis on the size effect of Al₂O₃ particles. The additions of Al₂O₃ particles retarded the densification and the effect was more remarkable with increasing Al₂O₃ volume fraction. The retardation effect of the Al₂O₃ dispersion particles was found to be independent of the particle size, and has been discussed from the following two aspects: (1) interactions between the dispersion particles that create a constraint on the matrix, and (2) formations of large pores or low-density regions around the dispersion particles due to non-uniform parking density in the mixed powder compacts.