In this study, we have modeled the rear side of the HVAC diffuser with square prism and tried to investigate the influence of a wake flow by PIV and a numerical analysis using computational fluid dynamics based on steady-state Navier-Stokes equation and standard k-ε model. A commercial CFD program, FLUENT, is used on the analysis. The wake is generated by a square prism, which is installed at the divergent flow. The results show that a fluid are distributed up and down directions of prism, and the recirculation regions are decreased.