Pure zirconium oxide powders with particle size 2-33 nm are synthesized by reactive plasma processing. Transmission electron microscopy investigation of these particles revealed size dependent behavior for their phase stabilization. The monoclinic phase is found to be stable when particle size is ³20 nm; Tetragonal is found to be stabilized in the range of 7-20 nm and as the particle size decreases to 6 nm  and less, the cubic phase is stabilized.