An image forming apparatus comprises a printing module, a transporting mechanism and a sorting module. The printing module prints data on media in order. The transporting mechanism transports the media past the printing module. The sorting module is disposed downstream of the transporting mechanism, and comprises an output mechanism, a rack and a driving pinion. The output mechanism transports the media out in a first direction. The rack is connected to the output mechanism. The driving pinion meshing with the rack drives the rack and the output mechanism to reciprocate in a second direction, different from the first direction, to sort the media into stacks. Thus, the sorting function can be achieved without providing a larger space and moving a discharge tray, and this is advantageous to the miniaturization of the image forming apparatus.