Our recent research on the dislocation behavior in GaN thin layers analyzed by means of cross sectional transmission electron microscopy (TEM) is briefly described. Layers of GaN and In_xGa_1-xN (x∼0.2) were grown by different vapor phase epitaxial methods. (1) On the surface of InGaN (T = 100 nm) grown on GaN, a pit was produced on the end of each threading dislocation. A thicker layer of InGaN has a two-story structure originated from the pits. This demonstrates that dislocations have strong influences on the growth process and the resultant layer-structure. (2) Microstructures in the epitaxial lateral overgrown (ELO) layers of GaN depend upon the growth conditions: When nitrogen (N₂) is used as a carrier gas, the c-axis of ELO-GaN tilts by 40° at most. The c-axis tilting is due to the generation of horizontal dislocations during the growth. The degree of tilting depends also on the width of mask-terrace. In case of a H₂-carrier gas, on the other hand, there is no tilting of c-axis, and regions of few dislocations are left over the terraces.