A study of polyethylene terephthalate (PET) and spectra fabric's ultrasonic sealing (welding) potential was conducted using both continuous and discontinuous ultrasonic welding machines. The effects of two important welding parameters such as weld pressure and weld time were investigated on the fabric's bond strength. Depending on the thermal property of the polymer, a certain level of temperature rise during sealing is required to soften the fibers, to facilitate the development of rows of welding points between the fabric layers. This happens under the compaction due to weld pressure. The gaps between the points are so narrow that the rows appear to be a complete sealing of the layers. Excessive melting under weld pressure, to create melt bonding caused polymer degradation and poor bond strength. Scanning Electron Microcopy images and the temperature measurements at the fabric's interface were used to examine the bond locations of the fabric. Differential Scanning Calorimetry analyses of PET and spectra fabrics have been used to examine the thermal behavior of the ultrasonic sealed material. Adequate seam strength was achieved under certain conditions of sealing for both the fabrics using both continuous and discontinuous methods of operation.