An experiment was conducted to investigate the mixing mechanism of a jet injected perpendicularly into a mainstream with a pseudo-shock wave (PSW). The velocity fields in the cross-sectional plane were obtained by particle-image velocimetry (PIV). Mixing of the injectant was observed by Mie scattering light of oil mist seeded in the injectant. The PSW thickened the boundary layer and reduced the momentum flux of the mainstream so the injectant penetrate higher from the wall. This affected the injectant mixing. The rd-scale model was applicable to estimate the injectant mixing in PSW by considering the flow condition just upstream of the injection jet. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.