The origin of the wake-up effect and the factor that determines the amount of the wake-up effect in ferroelectric (FE) HfO2 were systematically investigated. By observing both the waking-up and the phase transformation induced by electric field cycling on the same scale, we clarified that the wake-up effect in our FE-HfO2 system is dominated by the phase transformation induced by electric field cycling. Nevertheless, other mechanisms might be prominent in other FE-HfO2 systems. In our study, we found an anomalous structural distortion induced in both FE and none-FE HfO2 films. We hypothesized that structural distortion is the factor that governs the phase transformation to be induced by electric field cycling. We verified our hypothesis by conducting a series of experiments to clarify the factor that determines the amount of anomalous structural distortion, and that the amount of structural distortion would be the universal factor that determines the amount of the phase transformation. We found that the structural distortion is originating from the shrinkage in the volume of HfO2 films during crystallization due to the difference in the densities among the crystallized phases. Finally, we verified that the amount of structural distortion can be regarded as the universal factor that determines the amount of phase transformation to be induced by electric field cycling.