A thermal plasma process was used to synthesize nanosized zirconia and yttria-stabilized zirconia powders. This paper presents a new method on the synthesis of nanosized zirconia and yttria-stabilized zirconia powders from the carbonates (zirconium carbonate and/or yttrium carbonate). The products from this process using zirconium carbonate were mixtures of monoclinic and tetragonal phases of zirconia. The amount of the tetragonal phase in the mixture increased as the plasma torch power or plasma gas flow rate increased. This is attributed to the formation of metastable tetragonal phase by rapid quenching. With the addition of yttria precursor, two combinations of zirconia phases were formed depending on the yttria contents. Pure tetragonal phase was obtained at 3.8 mol% yttria, while a mixture of tetragonal and cubic phases was produced at 7.7 mol% yttria. The yttria-doped tetragonal phase was stable at high temperatures without the tetragonal-monoclinic phase transition. The grain size of each phase, the particle size, and the BET are also discussed in this paper.