An image forming system includes an image forming apparatus including a heating fixing portion and a host computer capable of instructing printing. In the image forming apparatus, throughput can be changed and discriminated in accordance with a printing number. For printing on small size sheets, the system is operable in a normal small size sheet mode and in a high speed small size sheet output mode, in which the printing is effected at a throughput which is higher than that in the normal small size sheet mode and, after completion of the printing, the image forming apparatus rests for a predetermined rest period. The host computer includes a mode selector for selecting a mode from the high speed small size sheet output mode and the normal small size sheet mode, and a controller for transmitting the mode selected by the mode selector to the image forming apparatus.