[en] A study was undertaken to examine the crystallite size effect on the low-temperature transformation of tetragonal zirconia. Zirconia was prepared by precipitation from a solution of zirconium tetrachloride by adding ammonium hydroxide to produce a pH of 2.95. Portions of the sample, after drying, were calcined at 500 degrees C for various time intervals. Phase transformation was followed by X-ray diffraction; the data show that the tetragonal phase was initially formed and it was transformed to the monoclinic phase at longer periods of calcination. It was observed by TEM particle size and XRD crystallite size that the transformation does not appear to be due to a critical particle size effect