Structural investigations by using high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) of silver nanodisks with different sizes are presented. The disks have a face centered cubic (fcc) crystal structure and their flat surfaces are (111). Stacking faults parallel to the (111) planes are frequently observed for the nanodisks. A unique (111) stacking fault model which is parallel to the flat (111) disk surface has been proposed to explain the observed 1/3{422} forbidden reflections in [111] SAED pattern and the corresponding 3 x {422} supperlattice fringes in the [111] HRTEM image. It is suggested that the presence of the stacking faults may be the key in the formation and growth of the disk morphology. This study may provide an insight to synthetically controlling particle shape and size through defect engineering.