Yttria stabilized zirconia (YSZ) nanocrystals were prepared by hydrothermal synthesis and were calcinated at different temperatures and with different isothermal hold time. X-ray diffraction via whole powder pattern modeling approach is used to study the grain growth as well as the evolution of grain size distribution of YSZ nanocrystals. Results show that YSZ nanocrystals start to grow along with the grain size distribution broadening at temperature about 300℃. The grain growth rate and the grain size distribution of YSZ nanocrystals are dependent on the calcination temperature and the isothermal hold time. The grain growth exponent and the active energy of grain growth were calculated. Grain rotation induced grain coalescence is suggested as the predominant way of grain growth of YSZ nanocrystals.