An image forming apparatus includes a housing, a drum, a toner processing module and a first mounting space. The drum is disposed in the housing. The toner processing module supplies toners to the drum. The first mounting space accommodates a first print head. The first print head provides a light ray to perform a writing operation on the drum. When the first print head is present, the light ray of the first print head disposed in the first mounting space passes through an optical channel of the toner processing module to perform the writing operation on the drum. Thus, the optimum configurations of the print head and the drum can be achieved, to effectively reduce a size of the image forming apparatus and satisfy diversified requirements without spending a high cost.