A Mach 2 rectangular nozzle with an aspect ratio 3 was used to investigate the effects of trailing edge modifications, which generate streamwise vorticity, on mixing enhancement at the fully expanded jet Mach numbers of 1.75, 2.0, and 2.5. The jet cross-sectional image was acquired by the laser sheet illumination technique. Surface flow visualizations and wall pressure measurements were used to investigate the major source of streamwise vorticity. Substantial mixing enhancement was achieved in the underexpanded flow condition. In this flow regime, the major source of streamwise vorticity was determined to be the spanwise surface pressure gradient on the modification. In the overexpanded flow regime, mixing enhancement was not substantial and the role of streamwise vorticity in mixing and the source of streamwise vorticity were unclear.