The nature of the atomic structure and the characteristics of both line and planar defects in decagonal quasicrystals of as‐cast Al62Cu20Co15Si3were studied by means of conventional diffraction contrast and high‐resolution electron microscopy. Two types of dislocations were observed. The magnitude and direction of the Burgers’ vector of type A dislocations and the Burgers’ vector direction of type B dislocations were determined. The direction of the displacement vector of the planar faults was defined.