The invention relates to ultrasonic gap welding of flat woven fabrics. Specifically, the invention relates to a method of ultrasonic welding the seam termination yarn ends of a fabric using a controlled gap. A mechanical stop may be employed to control the distance or gap between the horn and anvil to a predetermined level based on the depth of welding and amount of weld energy. By adding another degree of control, the ultrasonic gap welding technique of the present invention is able to achieve fabric seams that are stronger than conventional seams, and even those enhanced with gluing, and is conducive to formation of fabric seams by spot welding or along the termination yarn ends of the fabric.