In this work, we revealed the dynamic process of atomic structure transitions of ferroelectric (FE) Hf _0.5 Zr ^0.5 O ² (HZO) film across Curie temperature in spherical aberration corrected transmission electron microscope (Cs-TEM) with in-situ controlled heating and cooling. We have the following new observations: (1) A first-order transition between polar orthorhombic (o−) and nonpolar tetragonal (t−) phases occurs as heating up to , while the reverse process is triggered by cooling with obvious thermal hysteresis ; (2) Monoclinic (m−) phase contributes to the stability of o-structure by introducing local compressive stress; (3) The cooling t-to-o transition is identified as martensitic-like transformation, therefore increasing the phase change driving force would be helpful to FE enhancement. This work provides solid evidences on structure transitions near of HZO film at atomic-scale, laying foundation for understanding the physical nature of ferroelectricity in hafnium oxide materials.