A printing apparatus according to one aspect of this invention includes a sheet holding unit configured to hold an envelope, and a detection unit configured to detect an opening width between guides for guiding an envelope held by the sheet holding unit. The printing apparatus further includes an identifying unit configured to identify a flap size of the envelope based on the opening width detected by the detection unit and a size set for the envelope held by the sheet holding unit, and a printing unit configured to print an image on the envelope by shifting image data based on the flap size identified by the identifying unit.