Powders of zirconia doped with 8mol% REO1.5 (RE=Yb, Y, Gd, and Nd) were synthesized by using the sol-spray pyrolysis method. The effects of RE3+ ionic radius on monoclinic (m) phase content of powders were investigated. The XRD results showed that the metastable tetragonal (t) phase underwent a complete phase change to form m and cubic (c) phases after a heat treatment of 50-100h at 1450 degrees C. A cation size-dependent reduction in the m phase content of the REO1.5 partially stabilized ZrO2 powders was observed. The SEM-EDS analysis of the powders revealed that the RE3+ concentrations in the c phases decreased with decreasing the RE3+ ionic radius. These results indicate that the stabilizer concentration of the c phase can be attributed to RE3+ ionic radii, resulting in different m phase contents. Compared to the REO1.5 single doped ZrO2, the m phase contents of 8mol% (RE0.5Yb0.5)-ZrO2 (RE=Y, Gd, and Nd) powders were significantly reduced. The results prove that the stabilizer with a lower RE3+ size is effective in reducing the m phase content of partially stabilized ZrO2 after annealing at high temperatures.