Abstract This paper describes the characterization of ultra nanosized crystals as monoclinic, tetragonal, cubic or mixed phases of ZrO 2 . Nanopowders are produced using the microwave method, which allows control of the ratio of zirconium to yttrium to yield various crystallographic structures. The synthesized ultra nanosized particles have very broad XRD peaks. In particular, the similarity of the XRD peaks of tetragonal and cubic nanocrystals makes it difficult to distinguish between these two crystals. In this paper, tetragonal and cubic nanocrystals were clearly distinguished using infrared spectroscopy (IR). An infrared spectroscopic method to quantitatively analyse the percentage distribution between the two crystal phases was proposed. The results showed that pure zirconium dioxide was composed of 60 % monoclinic and 40 % tetragonal phases, while 8 % yttria‐stabilized zirconia (YSZ) contained 50 % tetragonal and 50 % cubic phases, and 14 % YSZ contained 100 % cubic phase.