An image forming system comprises an image forming apparatus for forming an image on a sheet and a sheet processing apparatus for performing binding processing on a sheet bundle formed of a plurality of sheets having formed the image. The sheet processing apparatus performs binding processing by a stapler on the sheet bundle placed on an intermediate tray and an opening. The image forming apparatus controls image forming preparation and timing of movement of the stapler in accordance with presence/absence of the sheet bundle on the opening, position of the stapler and timing to obtain an image forming instruction including binding processing.