The influence of grain size and yttria content on the hydrothermal aging behavior of yttria-stabilized zirconia ceramics has been investigated via the preparation of genuine, fully dense nanozirconia ceramics with <100 nm grain size by two-step sintering. The samples were aged at different temperatures for varying periods of time and XRD and micro-Raman spectroscopy have been used to monitor the aging process while the hardness of the samples was used as a measure of the deterioration in the physical properties. The results were compared with those of a commercial submicrometer 3YSZ zirconia sample subjected to the same aging treatment and characterization. Nano3YSZ with <100 nm grain size did not exhibit degradation, whereas the submicrometer 3YSZ samples underwent severe degradation.