An image forming apparatus which is capable of avoiding unnecessary switching operation to switch between a one-sided printing and a two-sided printing to improve print productivity. A printing mode is switched between a two-sided printing mode to continuously form images on first sides of a predetermined number of sheets and to alternately form images on second sides of sheets conveyed along a sheet re-feeding conveyance path and on the first sides of sheets fed from a sheet feeding unit, and a one-sided printing mode without using the sheet re-feeding conveyance path. It is determined that, when a one-sided print job is performed between a two-sided print jobs, whether the one-sided print job is performed in the two-sided printing mode as-is or in the one-sided printing mode, depending on whether the number of sheets continuously printed by the one-sided print job is more than a predetermined number.