An image transfer device to accurately measure a resistance of a printing medium or a transfer belt, an image forming apparatus having the image transfer device, and a method of measuring the resistance of the printing medium. The image transfer device includes a conveyance unit, to which a reference voltage or current is applied, and which includes a first roller having variations of resistance along a circumferential direction, and a second roller to support the first roller, a transfer roller spaced apart from the first roller by a distance corresponding to at least the circumference of the first roller to transfer a developer agent of a photosensitive drum onto a printing medium, and a controller to measure resistance of the printing medium by detecting electric current or voltage difference between the first and the second rollers, and to supply the transfer roller with a transfer voltage or a current according to the detected resistance of the printing medium.