We investigated in detail the microstructure of -oriented β-Ga2O3 thin films on sapphire substrates by transmission electron microscopy (TEM) and electron diffraction. Cross-sectional TEM images of the films on (001) c-plane and (110) a-plane sapphire substrates are composed of four types of regions with different atomic arrangements. The consideration based on the crystal orientation of β-Ga2O3 showed that the type of TEM images corresponds to the projection direction caused by the rotation of the β-Ga2O3 unit cell round the normal to the plane. Electron diffraction patterns corresponding to each region can also be explained in the same manner. These support the results of X-ray pole figure measurements in our previous report. Finally, the relationship among the TEM images, the electron diffraction patterns and the rotation angle of unit cell for -oriented β-Ga2O3 was classified.