Abstract: The PIV technique was utilized to study thecharacteristics of laminar unsteady horseshoe vortex in juncture flow. Thevisulization and the PIV show that there are three unsteady modes:amalgamating, shedding off-amalgamating and shedding off-dissipating. At acertain Reynolds number the shedding off vortex can be presented as sheddingoff-amalgamating or shedding off- dissipating, witch depend on the leadingedge shape of modals, for it changes the stretch of the vortices so that itinfluence the dissipation and diffusion of the corner vortex. The PIV showthat secondary vortex forms a growing negative vorticity tongue, which mayeventually penetrate into the boundary layer and effectively isolates theprimary vortex from the impinging B.L., allowing the primary vortex to shedoff. In unsteady oscillation mode, the vortex singular points show rathercomplex structure combination, which indicate the alternating stretch andcompression of the vortices.