An image forming apparatus including a photosensitive bodies fixing unit. The image forming apparatus includes: a main body frame; a fixing frame on which a plurality of photosensitive bodies are installed, the fixing frame being fixed at a predetermined position on the main body frame; a guide protruding portion formed on the fixing frame to guide the fixing frame to be coupled with the main body frame; a guide rail formed on the main body frame for the guide protruding portion to be inserted into; and a position determining portion formed on the guide rail to fix the guide protruding portion to a predetermined position. Accordingly, the parallel alignment between the scanning units and the photosensitive bodies may be relatively easily maintained and accurate color overlapping and color presentation are obtained to achieve high image quality. Further, the fixing frame may be fixed at a predetermined position by adjusting the tolerance of a part of the position determining portion, and the transfer unit may be integrated with the fixing frame and fixed at a predetermined position on the main body frame.