The microstructure of a wide gap ZnTe epilayer on a (100) GaAs substrate has been investigated in detail by transmission electron microscopy (TEM). Through high-resolution electron microscopy (HREM), extended defects such as dislocations and stacking faults have been clearly observed in the ZnTe epilayer near the interface. Considerable lattice misorientation in the local area of the epilayer was clarified, being consistent with the results of x-ray diffraction. Furthermore, it was found that the lattice contraction along the [200] direction and the lattice expansion along the [022] direction exist in the GaAs substrate. The lattice contraction and expansion were quantitatively analysed by advanced TEM, i.e., image analysis on HREM images and nanobeam electron diffraction.