An image forming apparatus and a fixing unit thereof are disclosed. The fixing unit includes a heating roller, a press roller mounted in close contact with the heating roller to apply heat and pressure to the printable medium with the heating roller, a press lever to press the press roller toward the heating roller, a press spring connected to the press lever, a release device to operate the press lever to release pressing force exerted on the press roller, and a power conversion device to transmit driving power generated from a main driving source to drive the heating roller to the release device. Accordingly, since the press lever pressing the press roller to closely contact the heating roller can be operated by the driving power of the main driving source, the jammed printable medium can be easily removed.