The defects in a synthesized crystal of ${alpha}-Al_2O_3$ used as substrate for growing of semi-conductor materials such as GaN were examined by the conventional transmission electron microscopy (TEM), Large Angle CBED and High-Angle Annular Dark Field (HAADF) STEM methods. The dominant defects found in the specimen are basal microtwins with the thickness of ${sim}2;to;32 nm$ and the associated strong strain field at the interface of microtwin/matrix, basal dislocations and complex dislocations in the one of {$2ar{1}ar{1}3$} pyramidal slip plane. All these basal and pyramidal dislocations seem to be strong related to basal microtwins. It was also found that the density of defects is very uneven. In the certain area with the dimension of a few fm, the dislocation density is quite high as an order of ${/sim}10^{10}/cm^2, but the average density is roughly estimated to be less than ${sim}10^5/cm^2, as is usually expected in general synthesized crystals.