CuO nanosheets (NSs) are synthesized by a one‐step template‐free solution method. The thermal characteristics are further accounted for by studying the morphological and structural evolution at different temperatures. The initial morphology and structure of the prepared CuO NSs are explored by scanning electron microscopy, energy dispersive spectrometer (EDS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X‐ray diffraction (XRD). These results confirm the advantages of smooth surface, and uniform and good crystalline array structure with a thickness of several tens of nanometers. During thermal analysis, in situ TEM heating is conducted from room temperature to 1120 °C, accompanied by the appearance and evolution of porous membrane areas and fusion areas. The changing process of shrinking and final disappearance is studied by in‐depth and detailed research. At the same time, in situ XRD heating from room temperature to 1100 °C and EDS analysis at 1020 °C provide auxiliary analysis of phase transition from CuO to Cu2O during in situ TEM heating. [ABSTRACT FROM AUTHOR]