Gallium oxide is considered as a perspective functional material for a wide range of applications. This includes light emitting devices, high power electronics, gas sensors and catalysts. Investigation of its structural features, particularly, defect structure of the gallium oxide epitaxial layers is crucial for the high quality devices production technology development. This work is focused on the investigation of the defect structure of thin epitaxial layers of (AlхGa1-х)2O3 possessing monoclinic structure grown by hydride vapor phase epitaxy on Al2O3 substrates. Some of the observed plane defects, twins and stacking faults, are shown for the first time.