The properties of InSb/InAs quantum dots (QDs) have been investigated by transmission electron microscopy (TEM). Specific features of diffraction contrast were discovered in plan-view TEM images of big (9–10 nm in height and 38–50 nm in diameter) InSb QDs. To understand the origin of such distortions in the TEM image the model of InSb QD on InAs substrate containing a partial Frank dislocation (FD)was developed and used for calculations of the displacement field and the subsequent diffraction image simulation of InSb QD for the first time. The shape of QD was established to influence insignificantly on the magnitude of radial displacements. The insertion of a misfit defect (a partial Frank dislocation) into QD reduces the strain at the edges of QD almost by 30%. The comparison of experimental and simulated data allowed us to explain the observed features of the moiré pattern in the image of big InSb QD by the presence of a misfit defect (FD) at the QD-substrate interface.