An image forming apparatus with a stapling function is provided. The image forming apparatus includes a printing device and a stapling device. The printing device is used for printing images on plural papers. The printed papers are transmitted to the stapling device and stapled by the stapling device. The printing device has a printing device casing. The stapling device includes a minor casing and a main casing, which are respectively connected with a first surface and a second surface of the printing device casing. Since the stapling device is connected with two difference surfaces of the printing device casing, the overall height of the image forming apparatus is reduced.