A fixing unit which enables high-speed operation and miniaturization, and an image forming apparatus having the fixing unit, includes a heating member which is heated by a heat source, the heating member having a predetermined width; a rotating member to rotate in contact with the heating member; a driving member to rotate the rotating member; and a pressing member to press both sides of the heating member towards the driving member and to form a predetermined fixing nip between the rotating member and the driving member, wherein the heating member has a second moment of inertia which is set to maintain a fixing efficiency of 90% or more in a central portion of the heating member.