Abstract: One of the most intriguing features of the Earth's inner core is its temporal variability over decadal time scale. Time-varying sesismic waves traversing the inner core, from high-quality repeating earthquakes, provide unequivocal evidence for the feature. However, the mechanism of the temporal change remains in debate. There have been two competing models, differential rotation of the inner core shifting its lateral heterogeneities and the rapid localized changes of the inner core boundary (ICB). Recent studies suggest that the time-varying seismic waves originate from the interior of the inner core, rather than the ICB. Consequently, the model of differential rotation provides so far the simplest and most reasonable explanation. Inner core is inferred to rotate at the rate of 0.05～0.1 degree per year, with the possibility of decadal fluctuation. The model can reconcile all the time-varying signals observed in previous studies. The investigations on the temporal change of the inner core and its mechanism shed lights on understanding the inner workings of our planet, especially the operation of the geodynamo.